linux-kernel - Re: [PATCH v2 3/4] sparc64: remove mm_cpumask clearing to fix kthread_use

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Message-Id: <1600138100.0flxk0qjzs.astroid@bobo.none>
Date:   Tue, 15 Sep 2020 12:49:17 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Anatoly Pugachev <matorola@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>,
        Jens Axboe <axboe@...nel.dk>,
        "David S . Miller" <davem@...emloft.net>,
        linux-arch@...r.kernel.org,
        Linux Kernel list <linux-kernel@...r.kernel.org>,
        linux-mm@...ck.org, linuxppc-dev@...ts.ozlabs.org,
        Peter Zijlstra <peterz@...radead.org>,
        Sparc kernel list <sparclinux@...r.kernel.org>
Subject: Re: [PATCH v2 3/4] sparc64: remove mm_cpumask clearing to fix
 kthread_use_mm race

Excerpts from Anatoly Pugachev's message of September 14, 2020 8:23 pm:
> On Mon, Sep 14, 2020 at 10:00 AM Nicholas Piggin <npiggin@...il.com> wrote:
>>
>> Excerpts from Nicholas Piggin's message of September 14, 2020 2:52 pm:
>>
>> [...]
>>
>> > The basic fix for sparc64 is to remove its mm_cpumask clearing code. The
>> > optimisation could be effectively restored by sending IPIs to mm_cpumask
>> > members and having them remove themselves from mm_cpumask. This is more
>> > tricky so I leave it as an exercise for someone with a sparc64 SMP.
>> > powerpc has a (currently similarly broken) example.
>>
>> So this compiles and boots on qemu, but qemu does not support any
>> sparc64 machines with SMP. Attempting some simple hacks doesn't get
>> me far because openbios isn't populating an SMP device tree, which
>> blows up everywhere.
>>
>> The patch is _relatively_ simple, hopefully it shouldn't explode, so
>> it's probably ready for testing on real SMP hardware, if someone has
>> a few cycles.
> 
> Nick,
> 
> applied this patch to over 'v5.9-rc5' tag , used my test VM (ldom)
> with 32 vcpus.
> Machine boot, stress-ng test ( run as
> "stress-ng --cpu 8 --io 8 --vm 8 --vm-bytes 2G --fork 8 --timeout 15m" )
> finishes without errors.
> 

Thank you very much Anatoly.

Thanks,
Nick