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Message-ID: <a6ee41b39f3f516aab0f7fb327620cb2a43eeaca.camel@redhat.com>
Subject: Re: [PATCH] mm/mmu_gather: send tlb_remove_table_smp_sync IPI only
 to MM CPUs
From:   ypodemsk@redhat.com
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     will@kernel.org, aneesh.kumar@linux.ibm.com,
        akpm@linux-foundation.org, npiggin@gmail.com, arnd@arndb.de,
        linux-arch@vger.kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, mtosatti@redhat.com,
        ppandit@redhat.com, alougovs@redhat.com,
        David Hildenbrand <david@redhat.com>
Date:   Wed, 22 Mar 2023 16:11:44 +0200
In-Reply-To: <20230320084902.GE2194297@hirez.programming.kicks-ass.net>
References: <20230312080945.14171-1-ypodemsk@redhat.com>
         <20230320084902.GE2194297@hirez.programming.kicks-ass.net>
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On Mon, 2023-03-20 at 09:49 +0100, Peter Zijlstra wrote:
> On Sun, Mar 12, 2023 at 10:09:45AM +0200, 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, this patch will limit this
> > IPI to
> > only be sent to cpus referencing the effected mm and are currently
> > in
> > kernel space.
> 
> Did you validate that all architectures for which this is relevant
> actually set bits in mm_cpumask() ?
> 
Hi Peter,
Thank you for bringing this to my attention.
I reviewed the architectures using the MMU_GATHER_RCU_TABLE_FREE:
arm, powerpc, s390, sparc and x86 set the bit when switching process
in.
for arm64 removed set/clear bit in 38d96287504a ("arm64: mm: kill
mm_cpumask usage")
The reason given was that mm_cpumask was not used.
Given that we now have a use for it, I will add a patch to revert.
Thanks
Yair