Re: [RFC PATCH] mm/page_alloc.c: micro-optimization reduce oom critical section size

[Mateusz Nosek]

On 9/14/2020 4:22 PM, Michal Hocko wrote:
> On Mon 14-09-20 12:06:54, mateusznosek0@xxxxxxxxx wrote:
> > From: Mateusz Nosek <mateusznosek0@xxxxxxxxx>
> >
> > 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.

Even despite the fact that optimization is 'micro' I think that code in 
question should be optimized, since it is possible without introducing 
additional problems, (assuming after verifying that it is safe as you 
mentioned below) and especially as it is in 'the heart' of memory 
allocation in kernel.

> Also I am not convinced the patch is entirely safe. At least the last
> allocation attempt is meant to be done under the lock to allow only one
> task to perform this. I have forgot the complete reasoning behind that
> but at least the changelog should argue why that is ok.

The last allocation just calls 'get_page_from_freelist' with two 
different sets of flags. I cannot see an obvious reason for it to 
require synchronization. I am investigating this deeper but if you 
happen to remember the reasoning or just any part of it and would be 
willing to share it, it would come with a lot of help.

> > Signed-off-by: Mateusz Nosek <mateusznosek0@xxxxxxxxx>
> > ---
> >   mm/page_alloc.c | 45 ++++++++++++++++++++++++---------------------
> >   1 file changed, 24 insertions(+), 21 deletions(-)
> >
> > 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);
> >   
> >   	/* 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