linux-kernel - Re: [PATCH 3/3] mm/mmu_gather: send tlb_remove_table_smp

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Date:   Thu, 06 Apr 2023 15:11:52 +0100
From:   Valentin Schneider <vschneid@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Frederic Weisbecker <frederic@...nel.org>,
        Yair Podemsky <ypodemsk@...hat.com>, linux@...linux.org.uk,
        mpe@...erman.id.au, npiggin@...il.com, christophe.leroy@...roup.eu,
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        linux-mm@...ck.org, mtosatti@...hat.com, dhildenb@...hat.com,
        alougovs@...hat.com
Subject: Re: [PATCH 3/3] mm/mmu_gather: send tlb_remove_table_smp_sync IPI
 only to CPUs in kernel mode

On 06/04/23 15:38, Peter Zijlstra wrote:
> On Wed, Apr 05, 2023 at 01:45:02PM +0100, Valentin Schneider wrote:
>>
>> I've been hacking on something like this (CSD deferral for NOHZ-full),
>> and unfortunately this uses the CPU-local cfd_data storage thing, which
>> means any further smp_call_function() from the same CPU to the same
>> destination will spin on csd_lock_wait(), waiting for the target CPU to
>> come out of userspace and flush the queue - and we've just spent extra
>> effort into *not* disturbing it, so that'll take a while :(
>
> I'm not sure I buy into deferring stuff.. a NOHZ_FULL cpu might 'never'
> come back. Queueing data just in case it does seems wasteful.

Putting those callbacks straight into the bin would make my life much
easier!

Unfortunately, even if they really should, I don't believe all of the
things being crammed onto NOHZ_FULL CPUs have the same definition of
'never' as we do :/