linux-kernel - Re: [RFC 1/1] mm: Add per-task struct tlb counters

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Date:   Wed, 14 Sep 2022 00:40:55 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Joe Damato <jdamato@...tly.com>, x86@...nel.org,
        linux-mm@...ck.org, Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [RFC 1/1] mm: Add per-task struct tlb counters

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.