linux-kernel - Re: [PATCH] mm: release the spinlock on zap_pte

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Date:   Mon, 29 Jul 2019 10:35:15 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Miguel de Dios <migueldedios@...gle.com>,
        Wei Wang <wvw@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Mel Gorman <mgorman@...hsingularity.net>
Subject: Re: [PATCH] mm: release the spinlock on zap_pte_range

On Mon 29-07-19 17:20:52, Minchan Kim wrote:
> On Mon, Jul 29, 2019 at 09:45:23AM +0200, Michal Hocko wrote:
> > On Mon 29-07-19 16:10:37, Minchan Kim wrote:
> > > In our testing(carmera recording), Miguel and Wei found unmap_page_range
> > > takes above 6ms with preemption disabled easily. When I see that, the
> > > reason is it holds page table spinlock during entire 512 page operation
> > > in a PMD. 6.2ms is never trivial for user experince if RT task couldn't
> > > run in the time because it could make frame drop or glitch audio problem.
> > 
> > Where is the time spent during the tear down? 512 pages doesn't sound
> > like a lot to tear down. Is it the TLB flushing?
> 
> Miguel confirmed there is no such big latency without mark_page_accessed
> in zap_pte_range so I guess it's the contention of LRU lock as well as
> heavy activate_page overhead which is not trivial, either.

Please give us more details ideally with some numbers.
-- 
Michal Hocko
SUSE Labs