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Date:   Wed, 14 Sep 2022 00:40:55 -0700
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Subject: Re: [RFC 1/1] mm: Add per-task struct tlb counters
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To:     Joe Damato <jdamato@fastly.com>, x86@kernel.org,
        linux-mm@kvack.org, Dave Hansen <dave.hansen@linux.intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Valentin Schneider <vschneid@redhat.com>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org
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 <1663120270-2673-2-git-send-email-jdamato@fastly.com>
From:   Dave Hansen <dave.hansen@intel.com>
In-Reply-To: <1663120270-2673-2-git-send-email-jdamato@fastly.com>
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On 9/13/22 18:51, Joe Damato wrote:
> TLB shootdowns are tracked globally, but on a busy system it can be
> difficult to disambiguate the source of TLB shootdowns.
> 
> Add two counter fields:
> 	- nrtlbflush: number of tlb flush events received
> 	- ngtlbflush: number of tlb flush events generated
> 
> Expose those fields in /proc/[pid]/stat so that they can be analyzed
> alongside similar metrics (e.g. min_flt and maj_flt).

On x86 at least, we already have two other ways to count flushes.  You
even quoted them with your patch:

>  	count_vm_tlb_event(NR_TLB_REMOTE_FLUSH);
> +	current->ngtlbflush++;
>  	if (info->end == TLB_FLUSH_ALL)
>  		trace_tlb_flush(TLB_REMOTE_SEND_IPI, TLB_FLUSH_ALL);

Granted, the count_vm_tlb...() one is debugging only.  But, did you try
to use those other mechanisms?  For instance, could you patch
count_vm_tlb_event()?  Why didn't the tracepoints work for you?

Can this be done in a more arch-generic way?  It's a shame to
unconditionally add counters to the task struct and only use them on
x86.  If someone wanted to generalize the x86 tracepoints, or make them
available to other architectures, I think that would be fine even if
they have to change a bit (queue the inevitable argument about
tracepoint ABI).

P.S. I'm not a fan of the structure member naming.