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Date:   Tue, 11 Dec 2018 09:53:52 -0800
From:   Andi Kleen <ak@linux.intel.com>
To:     Dave Hansen <dave.hansen@intel.com>
Cc:     Aubrey Li <aubrey.li@intel.com>, tglx@linutronix.de,
        mingo@redhat.com, peterz@infradead.org, hpa@zytor.com,
        tim.c.chen@linux.intel.com, arjan@linux.intel.com,
        linux-kernel@vger.kernel.org, Aubrey Li <aubrey.li@linux.intel.com>
Subject: Re: [PATCH v4 1/2] x86/fpu: track AVX-512 usage of tasks
Message-ID: <20181211175352.GC25620@tassilo.jf.intel.com>
References: <20181211002448.3520-1-aubrey.li@intel.com>
 <df1a1b5b-3e8b-5d96-12df-25be30c391af@intel.com>
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On Tue, Dec 11, 2018 at 09:18:25AM -0800, Dave Hansen wrote:
> 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?

Yes that's why we may need to propagate it to cgroups in the kernel,
because user daemons don't really want to track every TID.

But per pid is a start so that people can start experimenting with this.

Then with some experience fancier interfaces for it can be implemented.

-Andi