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SubjectRe: [RFC PATCH] mm/page_alloc.c: micro-optimization reduce oom critical section size
On Tue 15-09-20 15:09:59, Mateusz Nosek wrote:
>
>
> On 9/14/2020 4:22 PM, Michal Hocko wrote:
> > On Mon 14-09-20 12:06:54, mateusznosek0@gmail.com wrote:
> > > From: Mateusz Nosek <mateusznosek0@gmail.com>
> > >
> > > Most operations from '__alloc_pages_may_oom' do not require oom_mutex hold.
> > > Exception is 'out_of_memory'. The patch refactors '__alloc_pages_may_oom'
> > > to reduce critical section size and improve overall system performance.
> >
> > This is a real slow path. What is the point of optimizing it? Do you
> > have any numbers?
> >
>
> I agree that as this is the slow path, then the hard, complicated
> optimizations are not recommended. In my humble opinion introduced patch is
> not complex and does not decrease code readability or maintainability. In a
> nutshell I see no drawbacks of applying it.

This is clearly a matter of taste. I do not see a good reason to apply
it TBH. It is a claimed optimization without any numbers to back that
claim. It is also a tricky area so I am usually very careful to touch
this code without a strong reason. Others might feel differently of
course.

[...]

Anyway, I have only now looked closer at the patch...

> > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > > index b9bd75cacf02..b07f950a5825 100644
> > > --- a/mm/page_alloc.c
> > > +++ b/mm/page_alloc.c
> > > @@ -3935,18 +3935,7 @@ __alloc_pages_may_oom(gfp_t gfp_mask, unsigned int order,
> > > .order = order,
> > > };
> > > struct page *page;
> > > -
> > > - *did_some_progress = 0;
> > > -
> > > - /*
> > > - * Acquire the oom lock. If that fails, somebody else is
> > > - * making progress for us.
> > > - */
> > > - if (!mutex_trylock(&oom_lock)) {
> > > - *did_some_progress = 1;
> > > - schedule_timeout_uninterruptible(1);
> > > - return NULL;
> > > - }
> > > + bool success;
> > > /*
> > > * Go through the zonelist yet one more time, keep very high watermark
> > > @@ -3959,14 +3948,17 @@ __alloc_pages_may_oom(gfp_t gfp_mask, unsigned int order,
> > > ~__GFP_DIRECT_RECLAIM, order,
> > > ALLOC_WMARK_HIGH|ALLOC_CPUSET, ac);
> > > if (page)
> > > - goto out;
> > > + return page;
> > > +
> > > + /* Check if somebody else is making progress for us. */
> > > + *did_some_progress = mutex_is_locked(&oom_lock);

This is not only quite ugly but wrong as well. In general checking for a
lock state is racy unless the lock is taken somewhere up the call chain.

In this particular case it wouldn't be a big deal because an additional
retry (did_some_progress = 1) is not really critical. It would likely be
nicer to be deterministic here and not retry on all the early bailouts
regardless of the lock state.

> > > /* Coredumps can quickly deplete all memory reserves */
> > > if (current->flags & PF_DUMPCORE)
> > > - goto out;
> > > + return NULL;
> > > /* The OOM killer will not help higher order allocs */
> > > if (order > PAGE_ALLOC_COSTLY_ORDER)
> > > - goto out;
> > > + return NULL;
> > > /*
> > > * We have already exhausted all our reclaim opportunities without any
> > > * success so it is time to admit defeat. We will skip the OOM killer
> > > @@ -3976,12 +3968,12 @@ __alloc_pages_may_oom(gfp_t gfp_mask, unsigned int order,
> > > * The OOM killer may not free memory on a specific node.
> > > */
> > > if (gfp_mask & (__GFP_RETRY_MAYFAIL | __GFP_THISNODE))
> > > - goto out;
> > > + return NULL;
> > > /* The OOM killer does not needlessly kill tasks for lowmem */
> > > if (ac->highest_zoneidx < ZONE_NORMAL)
> > > - goto out;
> > > + return NULL;
> > > if (pm_suspended_storage())
> > > - goto out;
> > > + return NULL;
> > > /*
> > > * XXX: GFP_NOFS allocations should rather fail than rely on
> > > * other request to make a forward progress.
> > > @@ -3992,8 +3984,20 @@ __alloc_pages_may_oom(gfp_t gfp_mask, unsigned int order,
> > > * failures more gracefully we should just bail out here.
> > > */
> > > + /*
> > > + * Acquire the oom lock. If that fails, somebody else is
> > > + * making progress for us.
> > > + */
> > > + if (!mutex_trylock(&oom_lock)) {
> > > + *did_some_progress = 1;
> > > + schedule_timeout_uninterruptible(1);
> > > + return NULL;
> > > + }
> > > + success = out_of_memory(&oc);
> > > + mutex_unlock(&oom_lock);
> > > +
> > > /* Exhausted what can be done so it's blame time */
> > > - if (out_of_memory(&oc) || WARN_ON_ONCE(gfp_mask & __GFP_NOFAIL)) {
> > > + if (success || WARN_ON_ONCE(gfp_mask & __GFP_NOFAIL)) {
> > > *did_some_progress = 1;
> > > /*
> > > @@ -4004,8 +4008,7 @@ __alloc_pages_may_oom(gfp_t gfp_mask, unsigned int order,
> > > page = __alloc_pages_cpuset_fallback(gfp_mask, order,
> > > ALLOC_NO_WATERMARKS, ac);
> > > }
> > > -out:
> > > - mutex_unlock(&oom_lock);
> > > +
> > > return page;
> > > }
> > > --
> > > 2.20.1
> > >
> >
> Sincerely yours,
> Mateusz Nosek

--
Michal Hocko
SUSE Labs

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