linux-kernel - Re: [v7 PATCH 12/12] mm: vmscan: shrink deferred objects proportional to priority

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Date:   Thu, 11 Feb 2021 19:52:20 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Yang Shi <shy828301@...il.com>
Cc:     Roman Gushchin <guro@...com>, Kirill Tkhai <ktkhai@...tuozzo.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Dave Chinner <david@...morbit.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>,
        Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [v7 PATCH 12/12] mm: vmscan: shrink deferred objects proportional
 to priority

On 2/11/21 6:29 PM, Yang Shi wrote:
> On Thu, Feb 11, 2021 at 5:10 AM Vlastimil Babka <vbabka@...e.cz> wrote:
>> >       trace_mm_shrink_slab_start(shrinker, shrinkctl, nr,
>> >                                  freeable, delta, total_scan, priority);
>> > @@ -737,10 +708,9 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
>> >               cond_resched();
>> >       }
>> >
>> > -     if (next_deferred >= scanned)
>> > -             next_deferred -= scanned;
>> > -     else
>> > -             next_deferred = 0;
>> > +     next_deferred = max_t(long, (nr - scanned), 0) + total_scan;
>>
>> And here's the bias I think. Suppose we scanned 0 due to e.g. GFP_NOFS. We count
>> as newly deferred both the "delta" part of total_scan, which is fine, but also
>> the "nr >> priority" part, where we failed to our share of the "reduce
>> nr_deferred" work, but I don't think it means we should also increase
>> nr_deferred by that amount of failed work.
> 
> Here "nr" is the saved deferred work since the last scan, "scanned" is
> the scanned work in this round, total_scan is the *unscanned" work
> which is actually "total_scan - scanned" (total_scan is decreased by
> scanned in each loop). So, the logic is "decrease any scanned work
> from deferred then add newly unscanned work to deferred". IIUC this is
> what "deferred" means even before this patch.

Hm I thought the logic was "increase by any new work (delta) that wasn't done,
decrease by old deferred work that was done now". My examples with scanned = 0
and scanned = total_work (total_work before subtracting scanned from it) should
demonstrate that the logic is different with your patch.

>> OTOH if we succeed and scan exactly the whole goal, we are subtracting from
>> nr_deferred both the "nr >> priority" part, which is correct, but also delta,
>> which was new work, not deferred one, so that's incorrect IMHO as well.
> 
> I don't think so. The deferred comes from new work, why not dec new
> work from deferred?
> 
> And, the old code did:
> 
> if (next_deferred >= scanned)
>                 next_deferred -= scanned;
>         else
>                 next_deferred = 0;
> 
> IIUC, it also decreases the new work (the scanned includes both last
> deferred and new delata).

Yes, but in the old code, next_deferred starts as

nr = count_nr_deferred()...
total_scan = nr;
delta = ... // something based on freeable
total_scan += delta;
next_deferred = total_scan; // in the common case total_scan >= 0

... and that's "total_scan" before "scanned" is subtracted from it, so it
includes the new_work ("delta"), so then it's OK to do "next_deferred -= scanned";

I still think your formula is (unintentionally) changing the logic. You can also
look at it from different angle, it's effectively (without the max_t() part) "nr
- scanned + total_scan" where total_scan is actually "total_scan - scanned" as
you point your yourself. So "scanned" is subtracted twice? That can't be correct...

>> So the calculation should probably be something like this?
>>
>>         next_deferred = max_t(long, nr + delta - scanned, 0);
>>
>> Thanks,
>> Vlastimil
>>
>> > +     next_deferred = min(next_deferred, (2 * freeable));
>> > +
>> >       /*
>> >        * move the unused scan count back into the shrinker in a
>> >        * manner that handles concurrent updates.
>> >
>>
>