linux-kernel - Re: [PATCH v5 00/12] fold per-CPU vmstats remotely

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Date:   Tue, 14 Mar 2023 13:25:53 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     Christoph Lameter <cl@...ux.com>,
        Aaron Tomlin <atomlin@...mlin.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Russell King <linux@...linux.org.uk>,
        Huacai Chen <chenhuacai@...nel.org>,
        Heiko Carstens <hca@...ux.ibm.com>, x86@...nel.org,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH v5 00/12] fold per-CPU vmstats remotely

On Mon 13-03-23 13:25:07, Marcelo Tosatti wrote:
> This patch series addresses the following two problems:
> 
>     1. A customer provided some evidence which indicates that
>        the idle tick was stopped; albeit, CPU-specific vmstat
>        counters still remained populated.
> 
>        Thus one can only assume quiet_vmstat() was not
>        invoked on return to the idle loop. If I understand
>        correctly, I suspect this divergence might erroneously
>        prevent a reclaim attempt by kswapd. If the number of
>        zone specific free pages are below their per-cpu drift
>        value then zone_page_state_snapshot() is used to
>        compute a more accurate view of the aforementioned
>        statistic.  Thus any task blocked on the NUMA node
>        specific pfmemalloc_wait queue will be unable to make
>        significant progress via direct reclaim unless it is
>        killed after being woken up by kswapd
>        (see throttle_direct_reclaim())

I have hard time to follow the actual problem described above. Are you
suggesting that a lack of pcp vmstat counters update has led to
reclaim issues? What is the said "evidence"? Could you share more of the
story please?

>     2. With a SCHED_FIFO task that busy loops on a given CPU,
>        and kworker for that CPU at SCHED_OTHER priority,
>        queuing work to sync per-vmstats will either cause that
>        work to never execute, or stalld (i.e. stall daemon)
>        boosts kworker priority which causes a latency
>        violation

Why is that a problem? Out-of-sync stats shouldn't cause major problems.
Or can they?

Thanks!
-- 
Michal Hocko
SUSE Labs