[PATCH 0/4] riscv: tlb flush improvements

[PATCH 0/4] riscv: tlb flush improvements

[Alexandre Ghiti]

This series optimizes the tlb flushes on riscv which used to simply
flush the whole tlb whatever the size of the range to flush or the size
of the stride.

Patch 3 introduces a threshold that is microarchitecture specific and
will very likely be modified by vendors, not sure though which mechanism
we'll use to do that (dt? alternatives? vendor initialization code?).

Next steps would be to implement:
- svinval extension as Mayuresh did here [1]
- BATCHED_UNMAP_TLB_FLUSH (I'll wait for arm64 patchset to land)
- MMU_GATHER_RCU_TABLE_FREE
- MMU_GATHER_MERGE_VMAS

Any other idea welcome.

[1] https://lore.kernel.org/linux-riscv/20230623123849.1425805-1-mchitale@ventanamicro.com/

Alexandre Ghiti (4):
  riscv: Improve flush_tlb()
  riscv: Improve flush_tlb_range() for hugetlb pages
  riscv: Make __flush_tlb_range() loop over pte instead of flushing the
    whole tlb
  riscv: Improve flush_tlb_kernel_range()

 arch/riscv/include/asm/tlb.h      |  6 +-
 arch/riscv/include/asm/tlbflush.h | 12 ++--
 arch/riscv/mm/tlbflush.c          | 93 +++++++++++++++++++++++++++----
 3 files changed, 94 insertions(+), 17 deletions(-)

-- 
2.39.2

[PATCH 1/4] riscv: Improve flush_tlb()

[Alexandre Ghiti]

For now, flush_tlb() simply calls flush_tlb_mm() which results in a
flush of the whole TLB. So let's use mmu_gather fields to provide a more
fine-grained flush of the TLB.

Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
---
 arch/riscv/include/asm/tlb.h      | 6 +++++-
 arch/riscv/include/asm/tlbflush.h | 3 +++
 arch/riscv/mm/tlbflush.c          | 7 +++++++
 3 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/include/asm/tlb.h b/arch/riscv/include/asm/tlb.h
index 120bcf2ed8a8..3373dcf0f413 100644
--- a/arch/riscv/include/asm/tlb.h
+++ b/arch/riscv/include/asm/tlb.h
@@ -15,7 +15,11 @@ static void tlb_flush(struct mmu_gather *tlb);
 
 static inline void tlb_flush(struct mmu_gather *tlb)
 {
-	flush_tlb_mm(tlb->mm);
+	if (tlb->fullmm || tlb->need_flush_all)
+		flush_tlb_mm(tlb->mm);
+	else
+		flush_tlb_mm_range(tlb->mm, tlb->start, tlb->end,
+				   tlb_get_unmap_size(tlb));
 }
 
 #endif /* _ASM_RISCV_TLB_H */
diff --git a/arch/riscv/include/asm/tlbflush.h b/arch/riscv/include/asm/tlbflush.h
index a09196f8de68..f5c4fb0ae642 100644
--- a/arch/riscv/include/asm/tlbflush.h
+++ b/arch/riscv/include/asm/tlbflush.h
@@ -32,6 +32,8 @@ static inline void local_flush_tlb_page(unsigned long addr)
 #if defined(CONFIG_SMP) && defined(CONFIG_MMU)
 void flush_tlb_all(void);
 void flush_tlb_mm(struct mm_struct *mm);
+void flush_tlb_mm_range(struct mm_struct *mm, unsigned long start,
+			unsigned long end, unsigned int page_size);
 void flush_tlb_page(struct vm_area_struct *vma, unsigned long addr);
 void flush_tlb_range(struct vm_area_struct *vma, unsigned long start,
 		     unsigned long end);
@@ -52,6 +54,7 @@ static inline void flush_tlb_range(struct vm_area_struct *vma,
 }
 
 #define flush_tlb_mm(mm) flush_tlb_all()
+#define flush_tlb_mm_range(mm, start, end, page_size) flush_tlb_all()
 #endif /* !CONFIG_SMP || !CONFIG_MMU */
 
 /* Flush a range of kernel pages */
diff --git a/arch/riscv/mm/tlbflush.c b/arch/riscv/mm/tlbflush.c
index 77be59aadc73..fa03289853d8 100644
--- a/arch/riscv/mm/tlbflush.c
+++ b/arch/riscv/mm/tlbflush.c
@@ -132,6 +132,13 @@ void flush_tlb_mm(struct mm_struct *mm)
 	__flush_tlb_range(mm, 0, -1, PAGE_SIZE);
 }
 
+void flush_tlb_mm_range(struct mm_struct *mm,
+			unsigned long start, unsigned long end,
+			unsigned int page_size)
+{
+	__flush_tlb_range(mm, start, end - start, page_size);
+}
+
 void flush_tlb_page(struct vm_area_struct *vma, unsigned long addr)
 {
 	__flush_tlb_range(vma->vm_mm, addr, PAGE_SIZE, PAGE_SIZE);
-- 
2.39.2

[PATCH 2/4] riscv: Improve flush_tlb_range() for hugetlb pages

[Alexandre Ghiti]

flush_tlb_range() uses a fixed stride of PAGE_SIZE and in its current form,
when a hugetlb mapping needs to be flushed, flush_tlb_range() flushes the
whole tlb: so set a stride of the size of the hugetlb mapping in order to
only flush the hugetlb mapping.

Note that THPs are directly handled by flush_pmd_tlb_range().

Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
---
 arch/riscv/mm/tlbflush.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/mm/tlbflush.c b/arch/riscv/mm/tlbflush.c
index fa03289853d8..3e4acef1f6bc 100644
--- a/arch/riscv/mm/tlbflush.c
+++ b/arch/riscv/mm/tlbflush.c
@@ -3,6 +3,7 @@
 #include <linux/mm.h>
 #include <linux/smp.h>
 #include <linux/sched.h>
+#include <linux/hugetlb.h>
 #include <asm/sbi.h>
 #include <asm/mmu_context.h>
 
@@ -147,7 +148,14 @@ void flush_tlb_page(struct vm_area_struct *vma, unsigned long addr)
 void flush_tlb_range(struct vm_area_struct *vma, unsigned long start,
 		     unsigned long end)
 {
-	__flush_tlb_range(vma->vm_mm, start, end - start, PAGE_SIZE);
+	unsigned long stride_shift;
+
+	stride_shift = is_vm_hugetlb_page(vma) ?
+				huge_page_shift(hstate_vma(vma)) :
+				PAGE_SHIFT;
+
+	__flush_tlb_range(vma->vm_mm,
+			  start, end - start, 1 << stride_shift);
 }
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
 void flush_pmd_tlb_range(struct vm_area_struct *vma, unsigned long start,
-- 
2.39.2

[PATCH 3/4] riscv: Make __flush_tlb_range() loop over pte instead of flushing the whole tlb

[Alexandre Ghiti]

Currently, when the range to flush covers more than one page (a 4K page or
a hugepage), __flush_tlb_range() flushes the whole tlb. Flushing the whole
tlb comes with a greater cost than flushing a single entry so we should
flush single entries up to a certain threshold so that:
threshold * cost of flushing a single entry < cost of flushing the whole
tlb.

This threshold is microarchitecture dependent and can/should be
overwritten by vendors.

Co-developed-by: Mayuresh Chitale <mchitale@ventanamicro.com>
Signed-off-by: Mayuresh Chitale <mchitale@ventanamicro.com>
Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
---
 arch/riscv/mm/tlbflush.c | 41 ++++++++++++++++++++++++++++++++++++++--
 1 file changed, 39 insertions(+), 2 deletions(-)

diff --git a/arch/riscv/mm/tlbflush.c b/arch/riscv/mm/tlbflush.c
index 3e4acef1f6bc..de61ecaa218a 100644
--- a/arch/riscv/mm/tlbflush.c
+++ b/arch/riscv/mm/tlbflush.c
@@ -24,13 +24,48 @@ static inline void local_flush_tlb_page_asid(unsigned long addr,
 			: "memory");
 }
 
+/*
+ * Flush entire TLB if number of entries to be flushed is greater
+ * than the threshold below. Platforms may override the threshold
+ * value based on marchid, mvendorid, and mimpid.
+ */
+unsigned long tlb_flush_all_threshold __read_mostly = 64;
+
+static void local_flush_tlb_range_threshold_asid(unsigned long start,
+						 unsigned long size,
+						 unsigned long stride,
+						 unsigned long asid)
+{
+	u16 nr_ptes_in_range = DIV_ROUND_UP(size, stride);
+	int i;
+
+	if (nr_ptes_in_range > tlb_flush_all_threshold) {
+		if (asid != -1)
+			local_flush_tlb_all_asid(asid);
+		else
+			local_flush_tlb_all();
+		return;
+	}
+
+	for (i = 0; i < nr_ptes_in_range; ++i) {
+		if (asid != -1)
+			local_flush_tlb_page_asid(start, asid);
+		else
+			local_flush_tlb_page(start);
+		start += stride;
+	}
+}
+
 static inline void local_flush_tlb_range(unsigned long start,
 		unsigned long size, unsigned long stride)
 {
 	if (size <= stride)
 		local_flush_tlb_page(start);
-	else
+	else if (size == (unsigned long)-1)
 		local_flush_tlb_all();
+	else
+		local_flush_tlb_range_threshold_asid(start, size, stride, -1);
+
 }
 
 static inline void local_flush_tlb_range_asid(unsigned long start,
@@ -38,8 +73,10 @@ static inline void local_flush_tlb_range_asid(unsigned long start,
 {
 	if (size <= stride)
 		local_flush_tlb_page_asid(start, asid);
-	else
+	else if (size == (unsigned long)-1)
 		local_flush_tlb_all_asid(asid);
+	else
+		local_flush_tlb_range_threshold_asid(start, size, stride, asid);
 }
 
 static void __ipi_flush_tlb_all(void *info)
-- 
2.39.2

[PATCH 4/4] riscv: Improve flush_tlb_kernel_range()

[Alexandre Ghiti]

This function used to simply flush the whole tlb of all harts, be more
subtile and try to only flush the range.

The problem is that we can only use PAGE_SIZE as stride since we don't know
the size of the underlying mapping and then this function will be improved
only if the size of the region to flush is < threshold * PAGE_SIZE.

Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
---
 arch/riscv/include/asm/tlbflush.h | 11 +++++-----
 arch/riscv/mm/tlbflush.c          | 35 +++++++++++++++++++++++--------
 2 files changed, 32 insertions(+), 14 deletions(-)

diff --git a/arch/riscv/include/asm/tlbflush.h b/arch/riscv/include/asm/tlbflush.h
index f5c4fb0ae642..7426fdcd8ec5 100644
--- a/arch/riscv/include/asm/tlbflush.h
+++ b/arch/riscv/include/asm/tlbflush.h
@@ -37,6 +37,7 @@ void flush_tlb_mm_range(struct mm_struct *mm, unsigned long start,
 void flush_tlb_page(struct vm_area_struct *vma, unsigned long addr);
 void flush_tlb_range(struct vm_area_struct *vma, unsigned long start,
 		     unsigned long end);
+void flush_tlb_kernel_range(unsigned long start, unsigned long end);
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
 #define __HAVE_ARCH_FLUSH_PMD_TLB_RANGE
 void flush_pmd_tlb_range(struct vm_area_struct *vma, unsigned long start,
@@ -53,15 +54,15 @@ static inline void flush_tlb_range(struct vm_area_struct *vma,
 	local_flush_tlb_all();
 }
 
-#define flush_tlb_mm(mm) flush_tlb_all()
-#define flush_tlb_mm_range(mm, start, end, page_size) flush_tlb_all()
-#endif /* !CONFIG_SMP || !CONFIG_MMU */
-
 /* Flush a range of kernel pages */
 static inline void flush_tlb_kernel_range(unsigned long start,
 	unsigned long end)
 {
-	flush_tlb_all();
+	local_flush_tlb_all();
 }
 
+#define flush_tlb_mm(mm) flush_tlb_all()
+#define flush_tlb_mm_range(mm, start, end, page_size) flush_tlb_all()
+#endif /* !CONFIG_SMP || !CONFIG_MMU */
+
 #endif /* _ASM_RISCV_TLBFLUSH_H */
diff --git a/arch/riscv/mm/tlbflush.c b/arch/riscv/mm/tlbflush.c
index de61ecaa218a..07cfed83bec8 100644
--- a/arch/riscv/mm/tlbflush.c
+++ b/arch/riscv/mm/tlbflush.c
@@ -117,18 +117,27 @@ static void __flush_tlb_range(struct mm_struct *mm, unsigned long start,
 			      unsigned long size, unsigned long stride)
 {
 	struct flush_tlb_range_data ftd;
-	struct cpumask *cmask = mm_cpumask(mm);
-	unsigned int cpuid;
+	struct cpumask *cmask, full_cmask;
 	bool broadcast;
 
-	if (cpumask_empty(cmask))
-		return;
+	if (mm) {
+		unsigned int cpuid;
+
+		cmask = mm_cpumask(mm);
+		if (cpumask_empty(cmask))
+			return;
+
+		cpuid = get_cpu();
+		/* check if the tlbflush needs to be sent to other CPUs */
+		broadcast = cpumask_any_but(cmask, cpuid) < nr_cpu_ids;
+	} else {
+		cpumask_setall(&full_cmask);
+		cmask = &full_cmask;
+		broadcast = true;
+	}
 
-	cpuid = get_cpu();
-	/* check if the tlbflush needs to be sent to other CPUs */
-	broadcast = cpumask_any_but(cmask, cpuid) < nr_cpu_ids;
 	if (static_branch_unlikely(&use_asid_allocator)) {
-		unsigned long asid = atomic_long_read(&mm->context.id) & asid_mask;
+		unsigned long asid = mm ? atomic_long_read(&mm->context.id) & asid_mask : 0;
 
 		if (broadcast) {
 			if (riscv_use_ipi_for_rfence()) {
@@ -162,7 +171,8 @@ static void __flush_tlb_range(struct mm_struct *mm, unsigned long start,
 		}
 	}
 
-	put_cpu();
+	if (mm)
+		put_cpu();
 }
 
 void flush_tlb_mm(struct mm_struct *mm)
@@ -194,6 +204,13 @@ void flush_tlb_range(struct vm_area_struct *vma, unsigned long start,
 	__flush_tlb_range(vma->vm_mm,
 			  start, end - start, 1 << stride_shift);
 }
+
+void flush_tlb_kernel_range(unsigned long start,
+			    unsigned long end)
+{
+	__flush_tlb_range(NULL, start, end, PAGE_SIZE);
+}
+
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
 void flush_pmd_tlb_range(struct vm_area_struct *vma, unsigned long start,
 			unsigned long end)
-- 
2.39.2

Re: [PATCH 0/4] riscv: tlb flush improvements

[Conor Dooley]

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Hey Alex,

On Tue, Jul 11, 2023 at 09:54:30AM +0200, Alexandre Ghiti wrote:
> This series optimizes the tlb flushes on riscv which used to simply
> flush the whole tlb whatever the size of the range to flush or the size
> of the stride.
> 
> Patch 3 introduces a threshold that is microarchitecture specific and
> will very likely be modified by vendors, not sure though which mechanism
> we'll use to do that (dt? alternatives? vendor initialization code?).
> 
> Next steps would be to implement:
> - svinval extension as Mayuresh did here [1]
> - BATCHED_UNMAP_TLB_FLUSH (I'll wait for arm64 patchset to land)
> - MMU_GATHER_RCU_TABLE_FREE
> - MMU_GATHER_MERGE_VMAS
> 
> Any other idea welcome.
> 
> [1] https://lore.kernel.org/linux-riscv/20230623123849.1425805-1-mchitale@ventanamicro.com/
> 
> Alexandre Ghiti (4):
>   riscv: Improve flush_tlb()
>   riscv: Improve flush_tlb_range() for hugetlb pages

>   riscv: Make __flush_tlb_range() loop over pte instead of flushing the
>     whole tlb

The whole series does not build on nommu & this one adds a build warning
for regular builds:
+      1 ../arch/riscv/mm/tlbflush.c:32:15: warning: symbol 'tlb_flush_all_threshold' was not declared. Should it be static?

Cheers,
Conor.

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Re: [PATCH 0/4] riscv: tlb flush improvements

[Alexandre Ghiti]

Hi Conor,


On 12/07/2023 09:08, Conor Dooley wrote:
> Hey Alex,
>
> On Tue, Jul 11, 2023 at 09:54:30AM +0200, Alexandre Ghiti wrote:
>> This series optimizes the tlb flushes on riscv which used to simply
>> flush the whole tlb whatever the size of the range to flush or the size
>> of the stride.
>>
>> Patch 3 introduces a threshold that is microarchitecture specific and
>> will very likely be modified by vendors, not sure though which mechanism
>> we'll use to do that (dt? alternatives? vendor initialization code?).


@Conor any idea how to achieve this?


>>
>> Next steps would be to implement:
>> - svinval extension as Mayuresh did here [1]
>> - BATCHED_UNMAP_TLB_FLUSH (I'll wait for arm64 patchset to land)
>> - MMU_GATHER_RCU_TABLE_FREE
>> - MMU_GATHER_MERGE_VMAS
>>
>> Any other idea welcome.
>>
>> [1] https://lore.kernel.org/linux-riscv/20230623123849.1425805-1-mchitale@ventanamicro.com/
>>
>> Alexandre Ghiti (4):
>>    riscv: Improve flush_tlb()
>>    riscv: Improve flush_tlb_range() for hugetlb pages
>>    riscv: Make __flush_tlb_range() loop over pte instead of flushing the
>>      whole tlb
> The whole series does not build on nommu & this one adds a build warning
> for regular builds:
> +      1 ../arch/riscv/mm/tlbflush.c:32:15: warning: symbol 'tlb_flush_all_threshold' was not declared. Should it be static?
>
> Cheers,
> Conor.


I'll fix the nommu build, sorry about that. Weird I missed this warning, 
that's an LLVM build right? That variable will need to overwritten by 
the vendors, so that should not be static (but it will depend on what 
solution we implement).


Thanks,


Alex

Re: [PATCH 0/4] riscv: tlb flush improvements

[Conor Dooley]

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On Wed, Jul 12, 2023 at 05:18:00PM +0200, Alexandre Ghiti wrote:
> On 12/07/2023 09:08, Conor Dooley wrote:
> > On Tue, Jul 11, 2023 at 09:54:30AM +0200, Alexandre Ghiti wrote:
> > > This series optimizes the tlb flushes on riscv which used to simply
> > > flush the whole tlb whatever the size of the range to flush or the size
> > > of the stride.
> > > 
> > > Patch 3 introduces a threshold that is microarchitecture specific and
> > > will very likely be modified by vendors, not sure though which mechanism
> > > we'll use to do that (dt? alternatives? vendor initialization code?).
> 
> 
> @Conor any idea how to achieve this?

It's in my queue of things to look at, just been prioritising the
extension related stuff the last few days. Hopefully I'll have a chance
to think about this tomorrow.. Famous last words probably.

> > > Next steps would be to implement:
> > > - svinval extension as Mayuresh did here [1]
> > > - BATCHED_UNMAP_TLB_FLUSH (I'll wait for arm64 patchset to land)
> > > - MMU_GATHER_RCU_TABLE_FREE
> > > - MMU_GATHER_MERGE_VMAS
> > > 
> > > Any other idea welcome.
> > > 
> > > [1] https://lore.kernel.org/linux-riscv/20230623123849.1425805-1-mchitale@ventanamicro.com/
> > > 
> > > Alexandre Ghiti (4):
> > >    riscv: Improve flush_tlb()
> > >    riscv: Improve flush_tlb_range() for hugetlb pages
> > >    riscv: Make __flush_tlb_range() loop over pte instead of flushing the
> > >      whole tlb
> > The whole series does not build on nommu & this one adds a build warning
> > for regular builds:
> > +      1 ../arch/riscv/mm/tlbflush.c:32:15: warning: symbol 'tlb_flush_all_threshold' was not declared. Should it be static?
> > 
> > Cheers,
> > Conor.
> 
> 
> I'll fix the nommu build, sorry about that. Weird I missed this warning,
> that's an LLVM build right? That variable will need to overwritten by the
> vendors, so that should not be static (but it will depend on what solution
> we implement).

Just make it static until we actually have a vendor implementation of
this stuff please, since we don't know what that will look like yet.

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Re: [PATCH 0/4] riscv: tlb flush improvements

[Palmer Dabbelt]

On Wed, 12 Jul 2023 10:19:47 PDT (-0700), Conor Dooley wrote:
> On Wed, Jul 12, 2023 at 05:18:00PM +0200, Alexandre Ghiti wrote:
>> On 12/07/2023 09:08, Conor Dooley wrote:
>> > On Tue, Jul 11, 2023 at 09:54:30AM +0200, Alexandre Ghiti wrote:
>> > > This series optimizes the tlb flushes on riscv which used to simply
>> > > flush the whole tlb whatever the size of the range to flush or the size
>> > > of the stride.
>> > > 
>> > > Patch 3 introduces a threshold that is microarchitecture specific and
>> > > will very likely be modified by vendors, not sure though which mechanism
>> > > we'll use to do that (dt? alternatives? vendor initialization code?).
>> 
>> 
>> @Conor any idea how to achieve this?
>
> It's in my queue of things to look at, just been prioritising the
> extension related stuff the last few days. Hopefully I'll have a chance
> to think about this tomorrow.. Famous last words probably.
>
>> > > Next steps would be to implement:
>> > > - svinval extension as Mayuresh did here [1]
>> > > - BATCHED_UNMAP_TLB_FLUSH (I'll wait for arm64 patchset to land)
>> > > - MMU_GATHER_RCU_TABLE_FREE
>> > > - MMU_GATHER_MERGE_VMAS
>> > > 
>> > > Any other idea welcome.
>> > > 
>> > > [1] https://lore.kernel.org/linux-riscv/20230623123849.1425805-1-mchitale@ventanamicro.com/
>> > > 
>> > > Alexandre Ghiti (4):
>> > >    riscv: Improve flush_tlb()
>> > >    riscv: Improve flush_tlb_range() for hugetlb pages
>> > >    riscv: Make __flush_tlb_range() loop over pte instead of flushing the
>> > >      whole tlb
>> > The whole series does not build on nommu & this one adds a build warning
>> > for regular builds:
>> > +      1 ../arch/riscv/mm/tlbflush.c:32:15: warning: symbol 'tlb_flush_all_threshold' was not declared. Should it be static?
>> > 
>> > Cheers,
>> > Conor.
>> 
>> 
>> I'll fix the nommu build, sorry about that. Weird I missed this warning,
>> that's an LLVM build right? That variable will need to overwritten by the
>> vendors, so that should not be static (but it will depend on what solution
>> we implement).
>
> Just make it static until we actually have a vendor implementation of
> this stuff please, since we don't know what that will look like yet.

It's just a performance issue, right?  IIRC the SiFive errata wasn't 
actually based on how many TLB flushes happen, they're just broken in 
general so it was a probability thing.

If that's the case I agree we can just start with something arbitrary to 
start and then figure out how to set the tunable later.  It's probably 
going to be workload-specific too, so we'll probably end up with both a 
firmware default and a userspace override (maybe a sys entry or 
whatever).