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

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Message-Id: <C42F16F8-646B-448D-A098-E1A98637E28D@gmail.com>
Date:   Wed, 31 Mar 2021 09:36:04 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     Linux-MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <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>,
        Mel Gorman <mgorman@...e.de>
Subject: Re: [RFC] NUMA balancing: reduce TLB flush via delaying mapping on
 hint page fault



> On Mar 31, 2021, at 6:16 AM, Mel Gorman <mgorman@...e.de> wrote:
> 
> On Wed, Mar 31, 2021 at 07:20:09PM +0800, Huang, Ying wrote:
>> Mel Gorman <mgorman@...e.de> writes:
>> 
>>> On Mon, Mar 29, 2021 at 02:26:51PM +0800, Huang Ying wrote:
>>>> For 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 is high, so it's better to be avoided if possible.  In
>>>> fact, if we delay mapping the page in PTE until migration, that can be
>>>> avoided.  This is what this patch doing.
>>>> 
>>> 
>>> Why would the overhead be high? It was previously inaccessibly so it's
>>> only parallel accesses making forward progress that trigger the need
>>> for a flush.
>> 
>> Sorry, I don't understand this.  Although the page is inaccessible, the
>> threads may access other pages, so TLB flushing is still necessary.
>> 
> 
> You assert the overhead of TLB shootdown is high and yes, it can be
> very high but you also said "the benchmark score has no visible changes"
> indicating the TLB shootdown cost is not a major problem for the workload.
> It does not mean we should ignore it though.

If you are looking for a benchmark that is negatively affected by NUMA
balancing, then IIRC Parsec’s dedup is such a workload. [1]

[1] https://parsec.cs.princeton.edu/

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