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Subject: Re: [PATCH v4 1/2] x86/fpu: track AVX-512 usage of tasks
To:     Dave Hansen <dave.hansen@intel.com>,
        Aubrey Li <aubrey.li@intel.com>, tglx@linutronix.de,
        mingo@redhat.com, peterz@infradead.org, hpa@zytor.com
Cc:     ak@linux.intel.com, arjan@linux.intel.com,
        linux-kernel@vger.kernel.org, Aubrey Li <aubrey.li@linux.intel.com>
References: <20181211002448.3520-1-aubrey.li@intel.com>
 <df1a1b5b-3e8b-5d96-12df-25be30c391af@intel.com>
From:   Tim Chen <tim.c.chen@linux.intel.com>
Message-ID: <86ba7095-b877-cea6-3c07-8db2ff7fe02f@linux.intel.com>
Date:   Tue, 11 Dec 2018 09:52:43 -0800
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On 12/11/18 9:18 AM, 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?
> 

The reason why I believe that an exponential decay average, or some
other kind of weighted decay of the AVX512 usage is better.

Tim