linux-kernel - Re: [PATCH v4 1/2] x86/fpu: track AVX-512 usage of tasks

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Message-ID: <df1a1b5b-3e8b-5d96-12df-25be30c391af@intel.com>
Date:   Tue, 11 Dec 2018 09:18:25 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Aubrey Li <aubrey.li@...el.com>, tglx@...utronix.de,
        mingo@...hat.com, peterz@...radead.org, hpa@...or.com
Cc:     ak@...ux.intel.com, tim.c.chen@...ux.intel.com,
        arjan@...ux.intel.com, linux-kernel@...r.kernel.org,
        Aubrey Li <aubrey.li@...ux.intel.com>
Subject: Re: [PATCH v4 1/2] x86/fpu: track AVX-512 usage of tasks

On 12/10/18 4:24 PM, Aubrey Li wrote:
> The tracking turns on the usage flag at the next context switch of
> the task, but requires 3 consecutive context switches with no usage
> to clear it. This decay is required because well-written AVX-512
> applications are expected to clear this state when not actively using
> AVX-512 registers.

One concern about this:  Given a HZ=1000 system, this means that the
flag needs to get scanned every ~3ms.  That's a pretty good amount of
scanning on a system with hundreds or thousands of tasks running around.

How many tasks does this scale to until you're eating up an entire CPU
or two just scanning /proc?