Re: [v2 PATCH] mm: mmu_gather: remove __tlb_reset_range() for force flush

[Yang Shi <yang.shi@linux.alibaba.com>]

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On 5/13/19 7:01 PM, Nadav Amit wrote:
>
>
> On May 13, 2019 4:01 PM, Yang Shi <yang.shi@linux.alibaba.com> wrote:
>
>
>
>     On 5/13/19 9:38 AM, Will Deacon wrote:
>     > On Fri, May 10, 2019 at 07:26:54AM +0800, Yang Shi wrote:
>     >> diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c
>     >> index 99740e1..469492d 100644
>     >> --- a/mm/mmu_gather.c
>     >> +++ b/mm/mmu_gather.c
>     >> @@ -245,14 +245,39 @@ void tlb_finish_mmu(struct mmu_gather *tlb,
>     >>   {
>     >>       /*
>     >>        * If there are parallel threads are doing PTE changes on
>     same range
>     >> -     * under non-exclusive lock(e.g., mmap_sem read-side) but
>     defer TLB
>     >> -     * flush by batching, a thread has stable TLB entry can
>     fail to flush
>     >> -     * the TLB by observing pte_none|!pte_dirty, for example
>     so flush TLB
>     >> -     * forcefully if we detect parallel PTE batching threads.
>     >> +     * under non-exclusive lock (e.g., mmap_sem read-side) but
>     defer TLB
>     >> +     * flush by batching, one thread may end up seeing
>     inconsistent PTEs
>     >> +     * and result in having stale TLB entries.  So flush TLB
>     forcefully
>     >> +     * if we detect parallel PTE batching threads.
>     >> +     *
>     >> +     * However, some syscalls, e.g. munmap(), may free page
>     tables, this
>     >> +     * needs force flush everything in the given range.
>     Otherwise this
>     >> +     * may result in having stale TLB entries for some
>     architectures,
>     >> +     * e.g. aarch64, that could specify flush what level TLB.
>     >>        */
>     >> -    if (mm_tlb_flush_nested(tlb->mm)) {
>     >> -            __tlb_reset_range(tlb);
>     >> -            __tlb_adjust_range(tlb, start, end - start);
>     >> +    if (mm_tlb_flush_nested(tlb->mm) && !tlb->fullmm) {
>     >> +            /*
>     >> +             * Since we can't tell what we actually should have
>     >> +             * flushed, flush everything in the given range.
>     >> +             */
>     >> +            tlb->freed_tables = 1;
>     >> +            tlb->cleared_ptes = 1;
>     >> +            tlb->cleared_pmds = 1;
>     >> +            tlb->cleared_puds = 1;
>     >> +            tlb->cleared_p4ds = 1;
>     >> +
>     >> +            /*
>     >> +             * Some architectures, e.g. ARM, that have range
>     invalidation
>     >> +             * and care about VM_EXEC for I-Cache
>     invalidation, need force
>     >> +             * vma_exec set.
>     >> +             */
>     >> +            tlb->vma_exec = 1;
>     >> +
>     >> +            /* Force vma_huge clear to guarantee safer flush */
>     >> +            tlb->vma_huge = 0;
>     >> +
>     >> +            tlb->start = start;
>     >> +            tlb->end = end;
>     >>       }
>     > Whilst I think this is correct, it would be interesting to see
>     whether
>     > or not it's actually faster than just nuking the whole mm, as I
>     mentioned
>     > before.
>     >
>     > At least in terms of getting a short-term fix, I'd prefer the
>     diff below
>     > if it's not measurably worse.
>
>     I did a quick test with ebizzy (96 threads with 5 iterations) on
>     my x86
>     VM, it shows slightly slowdown on records/s but much more sys time
>     spent
>     with fullmm flush, the below is the data.
>
>     nofullmm                 fullmm
>     ops (records/s) 225606                  225119
>     sys (s) 0.69                        1.14
>
>     It looks the slight reduction of records/s is caused by the
>     increase of
>     sys time.
>
>     >
>     > Will
>     >
>     > --->8
>     >
>     > diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c
>     > index 99740e1dd273..cc251422d307 100644
>     > --- a/mm/mmu_gather.c
>     > +++ b/mm/mmu_gather.c
>     > @@ -251,8 +251,9 @@ void tlb_finish_mmu(struct mmu_gather *tlb,
>     >         * forcefully if we detect parallel PTE batching threads.
>     >         */
>     >        if (mm_tlb_flush_nested(tlb->mm)) {
>     > +             tlb->fullmm = 1;
>     >                __tlb_reset_range(tlb);
>     > -             __tlb_adjust_range(tlb, start, end - start);
>     > +             tlb->freed_tables = 1;
>     >        }
>     >
>     >        tlb_flush_mmu(tlb);
>
>
> I think that this should have set need_flush_all and not fullmm.

Thanks for the suggestion. I did a quick test with ebizzy too. It looks 
this is almost same with the v2 patch and slightly better than what Will 
suggested.

nofullmm                 fullmm                need_flush_all
ops (records/s)              225606 225119                   225647
sys (s)                            0.69 1.14                          0.47

If no objection from other folks, I would respin the patch based off 
Nadav's suggestion.




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