Re: in_irq_or_nmi() and RFC patch

[Mel Gorman <mgorman@techsingularity.net>]

On Wed, Mar 29, 2017 at 09:44:41PM +0200, Jesper Dangaard Brouer wrote:
> > Regardless or using in_irq() (or in combi with in_nmi()) I get the
> > following warning below:
> > 
> > [    0.000000] Kernel command line: BOOT_IMAGE=/vmlinuz-4.11.0-rc3-net-next-page-alloc-softirq+ root=UUID=2e8451ff-6797-49b5-8d3a-eed5a42d7dc9 ro rhgb quiet LANG=en_DK.UTF
> > -8
> > [    0.000000] PID hash table entries: 4096 (order: 3, 32768 bytes)
> > [    0.000000] ------------[ cut here ]------------
> > [    0.000000] WARNING: CPU: 0 PID: 0 at kernel/softirq.c:161 __local_bh_enable_ip+0x70/0x90
> > [    0.000000] Modules linked in:
> > [    0.000000] CPU: 0 PID: 0 Comm: swapper Not tainted 4.11.0-rc3-net-next-page-alloc-softirq+ #235
> > [    0.000000] Hardware name: MSI MS-7984/Z170A GAMING PRO (MS-7984), BIOS 1.60 12/16/2015
> > [    0.000000] Call Trace:
> > [    0.000000]  dump_stack+0x4f/0x73
> > [    0.000000]  __warn+0xcb/0xf0
> > [    0.000000]  warn_slowpath_null+0x1d/0x20
> > [    0.000000]  __local_bh_enable_ip+0x70/0x90
> > [    0.000000]  free_hot_cold_page+0x1a4/0x2f0
> > [    0.000000]  __free_pages+0x1f/0x30
> > [    0.000000]  __free_pages_bootmem+0xab/0xb8
> > [    0.000000]  __free_memory_core+0x79/0x91
> > [    0.000000]  free_all_bootmem+0xaa/0x122
> > [    0.000000]  mem_init+0x71/0xa4
> > [    0.000000]  start_kernel+0x1e5/0x3f1
> > [    0.000000]  x86_64_start_reservations+0x2a/0x2c
> > [    0.000000]  x86_64_start_kernel+0x178/0x18b
> > [    0.000000]  start_cpu+0x14/0x14
> > [    0.000000]  ? start_cpu+0x14/0x14
> > [    0.000000] ---[ end trace a57944bec8fc985c ]---
> > [    0.000000] Memory: 32739472K/33439416K available (7624K kernel code, 1528K rwdata, 3168K rodata, 1860K init, 2260K bss, 699944K reserved, 0K cma-reserved)
> > 
> > And kernel/softirq.c:161 contains:
> > 
> >  WARN_ON_ONCE(in_irq() || irqs_disabled());
> > 
> > Thus, I don't think the change in my RFC-patch[1] is safe.
> > Of changing[2] to support softirq allocations by replacing
> > preempt_disable() with local_bh_disable().
> > 
> > [1] http://lkml.kernel.org/r/20170327143947.4c237e54@redhat.com
> > 
> > [2] commit 374ad05ab64d ("mm, page_alloc: only use per-cpu allocator for irq-safe requests")
> >  https://git.kernel.org/torvalds/c/374ad05ab64d
> 
> A patch that avoids the above warning is inlined below, but I'm not
> sure if this is best direction.  Or we should rather consider reverting
> part of commit 374ad05ab64d to avoid the softirq performance regression?
>  

At the moment, I'm not seeing a better alternative. If this works, I
think it would still be far superior in terms of performance than a
revert. As before, if there are bad consequences to adding a BH
rescheduling point then we'll have to revert. However, I don't like a
revert being the first option as it'll keep encouraging drivers to build
sub-allocators to avoid the page allocator.

> [PATCH] mm, page_alloc: re-enable softirq use of per-cpu page allocator
> 
> From: Jesper Dangaard Brouer <brouer@redhat.com>
> 

Other than the slightly misleading comments about NMI which could
explain "this potentially misses an NMI but an NMI allocating pages is
brain damaged", I don't see a problem. The irqs_disabled() check is a
subtle but it's not earth shattering and it still helps the 100GiB cases
with the limited cycle budget to process packets.

-- 
Mel Gorman
SUSE Labs

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