linux-kernel - Re: [PATCH v2 0/2] send tlb_remove_table_smp

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Message-ID: <20230703140950.GL4253@hirez.programming.kicks-ass.net>
Date:   Mon, 3 Jul 2023 16:09:50 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     Yair Podemsky <ypodemsk@...hat.com>, ppandit@...hat.com,
        david@...hat.com, linux@...linux.org.uk, mpe@...erman.id.au,
        npiggin@...il.com, christophe.leroy@...roup.eu, hca@...ux.ibm.com,
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Subject: Re: [PATCH v2 0/2] send tlb_remove_table_smp_sync IPI only to
 necessary CPUs

On Thu, Jun 22, 2023 at 09:47:22AM -0300, Marcelo Tosatti wrote:

> > there's patches that cure the thp abuse of this.
> 
> The other case where the IPI can happen is:
> 
> CPU-0                                   CPU-1
> 
> tlb_remove_table
> tlb_remove_table_sync_one
> IPI
>                                         local_irq_disable
>                                         gup_fast
>                                         local_irq_enable
> 
> 
> So its not only the THP case.

(your CPU-1 thing is wholly irrelevant)

Well, I know, but this case *should* be exceedingly rare. Last time
around I asked why you all were tripping this, you pointed at the THP
case.

The THP case should be fixed along the lines of Jann's original patches.

If you can trip this at any significant rate, then we should probably
look at a better allocation scheme. It means you're really low on
memory.