linux-kernel - Re: [PATCH 6/6] mm/page_alloc: Remotely drain per-cpu lists

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <20220512123743.5be26b3ad4413f20d5f46564@linux-foundation.org>
Date:   Thu, 12 May 2022 12:37:43 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Nicolas Saenz Julienne <nsaenzju@...hat.com>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Michal Hocko <mhocko@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH 6/6] mm/page_alloc: Remotely drain per-cpu lists

On Thu, 12 May 2022 09:50:43 +0100 Mel Gorman <mgorman@...hsingularity.net> wrote:

> From: Nicolas Saenz Julienne <nsaenzju@...hat.com>
> 
> Some setups, notably NOHZ_FULL CPUs, are too busy to handle the per-cpu
> drain work queued by __drain_all_pages(). So introduce a new mechanism to
> remotely drain the per-cpu lists. It is made possible by remotely locking
> 'struct per_cpu_pages' new per-cpu spinlocks. A benefit of this new scheme
> is that drain operations are now migration safe.
> 
> There was no observed performance degradation vs. the previous scheme.
> Both netperf and hackbench were run in parallel to triggering the
> __drain_all_pages(NULL, true) code path around ~100 times per second.
> The new scheme performs a bit better (~5%), although the important point
> here is there are no performance regressions vs. the previous mechanism.
> Per-cpu lists draining happens only in slow paths.
> 
> Minchan Kim tested this independently and reported;
> 
> 	My workload is not NOHZ CPUs but run apps under heavy memory
> 	pressure so they goes to direct reclaim and be stuck on
> 	drain_all_pages until work on workqueue run.
> 
> 	unit: nanosecond
> 	max(dur)        avg(dur)                count(dur)
> 	166713013       487511.77786438033      1283
> 
> 	From traces, system encountered the drain_all_pages 1283 times and
> 	worst case was 166ms and avg was 487us.
> 
> 	The other problem was alloc_contig_range in CMA. The PCP draining
> 	takes several hundred millisecond sometimes though there is no
> 	memory pressure or a few of pages to be migrated out but CPU were
> 	fully booked.
> 
> 	Your patch perfectly removed those wasted time.

I'm not getting a sense here of the overall effect upon userspace
performance.  As Thomas said last year in
https://lkml.kernel.org/r/87v92sgt3n.ffs@tglx

: The changelogs and the cover letter have a distinct void vs. that which
: means this is just another example of 'scratch my itch' changes w/o
: proper justification.

Is there more to all of this than itchiness and if so, well, you know
the rest ;)