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Date:   Thu, 22 Jun 2023 09:47:22 -0300
From:   Marcelo Tosatti <mtosatti@redhat.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Yair Podemsky <ypodemsk@redhat.com>, ppandit@redhat.com,
        david@redhat.com, linux@armlinux.org.uk, mpe@ellerman.id.au,
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Subject: Re: [PATCH v2 0/2] send tlb_remove_table_smp_sync IPI only to
 necessary CPUs
Message-ID: <ZJRC2s4sIuJ9V3A0@tpad>
References: <20230620144618.125703-1-ypodemsk@redhat.com>
 <20230621074337.GF2046280@hirez.programming.kicks-ass.net>
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On Wed, Jun 21, 2023 at 09:43:37AM +0200, Peter Zijlstra wrote:
> On Tue, Jun 20, 2023 at 05:46:16PM +0300, Yair Podemsky wrote:
> > Currently the tlb_remove_table_smp_sync IPI is sent to all CPUs
> > indiscriminately, this causes unnecessary work and delays notable in
> > real-time use-cases and isolated cpus.
> > By limiting the IPI to only be sent to cpus referencing the effected
> > mm.
> > a config to differentiate architectures that support mm_cpumask from
> > those that don't will allow safe usage of this feature.
> > 
> > changes from -v1:
> > - Previous version included a patch to only send the IPI to CPU's with
> > context_tracking in the kernel space, this was removed due to race 
> > condition concerns.
> > - for archs that do not maintain mm_cpumask the mask used should be
> >  cpu_online_mask (Peter Zijlstra).
> >  
> 
> Would it not be much better to fix the root cause? As per the last time,
> 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.