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

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Message-ID: <ZJRC2s4sIuJ9V3A0@tpad>
Date:   Thu, 22 Jun 2023 09:47:22 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
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,
        gor@...ux.ibm.com, agordeev@...ux.ibm.com,
        borntraeger@...ux.ibm.com, svens@...ux.ibm.com,
        davem@...emloft.net, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, dave.hansen@...ux.intel.com, hpa@...or.com,
        keescook@...omium.org, paulmck@...nel.org, frederic@...nel.org,
        will@...nel.org, ardb@...nel.org, samitolvanen@...gle.com,
        juerg.haefliger@...onical.com, arnd@...db.de,
        rmk+kernel@...linux.org.uk, geert+renesas@...der.be,
        linus.walleij@...aro.org, akpm@...ux-foundation.org,
        sebastian.reichel@...labora.com, rppt@...nel.org,
        aneesh.kumar@...ux.ibm.com, x86@...nel.org,
        linux-arm-kernel@...ts.infradead.org,
        linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
        sparclinux@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/2] send tlb_remove_table_smp_sync IPI only to
 necessary CPUs

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.