linux-kernel - Re: [PATCH -V2] NUMA balancing: reduce TLB flush via delaying mapping on hint page fault

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Message-ID: <87lf9syjmk.fsf@yhuang6-desk1.ccr.corp.intel.com>
Date:   Thu, 08 Apr 2021 21:46:11 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Mel Gorman <mgorman@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Peter Xu" <peterx@...hat.com>,
        Johannes Weiner <hannes@...xchg.org>,
        "Vlastimil Babka" <vbabka@...e.cz>,
        Matthew Wilcox <willy@...radead.org>,
        Will Deacon <will@...nel.org>,
        Michel Lespinasse <walken@...gle.com>,
        Arjun Roy <arjunroy@...gle.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH -V2] NUMA balancing: reduce TLB flush via delaying
 mapping on hint page fault

Mel Gorman <mgorman@...e.de> writes:

> On Fri, Apr 02, 2021 at 04:27:17PM +0800, Huang Ying wrote:
>> With NUMA balancing, in hint page fault handler, the faulting page
>> will be migrated to the accessing node if necessary.  During the
>> migration, TLB will be shot down on all CPUs that the process has run
>> on recently.  Because in the hint page fault handler, the PTE will be
>> made accessible before the migration is tried.  The overhead of TLB
>> shooting down can be high, so it's better to be avoided if possible.
>> In fact, if we delay mapping the page until migration, that can be
>> avoided.  This is what this patch doing.
>> 
>> <SNIP>
>>
>
> Thanks, I think this is ok for Andrew to pick up to see if anything
> bisects to this commit but it's a low risk.
>
> Reviewed-by: Mel Gorman <mgorman@...e.de>
>
> More notes;
>
> This is not a universal win given that not all workloads exhibit the
> pattern where accesses occur in parallel threads between when a page
> is marked accessible and when it is migrated. The impact of the patch
> appears to be neutral for those workloads. For workloads that do exhibit
> the pattern, there is a small gain with a reduction in interrupts as
> advertised unlike v1 of the patch. Further tests are running to confirm
> the reduction is in TLB shootdown interrupts but I'm reasonably confident
> that will be the case. Gains are typically small and the load described in
> the changelog appears to be a best case scenario but a 1-5% gain in some
> other workloads is still an improvement. There is still the possibility
> that some workloads will unnecessarily stall as a result of the patch
> for slightly longer periods of time but that is a relatively low risk
> and will be difficult to detect. If I'm wrong, a bisection will find it.

Hi, Mel,

Thanks!

Hi, Andrew,

I found that V2 cannot apply on top of latest mmotm, so I send V3 as
follows.  In case you need it.

https://lore.kernel.org/lkml/20210408132236.1175607-1-ying.huang@intel.com/

Best Regards,
Huang, Ying