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Date:   Thu, 19 Jul 2018 11:26:14 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Tejun Heo <tj@kernel.org>,
        Suren Baghdasaryan <surenb@google.com>,
        Vinayak Menon <vinmenon@codeaurora.org>,
        Christopher Lameter <cl@linux.com>,
        Mike Galbraith <efault@gmx.de>,
        Shakeel Butt <shakeelb@google.com>, linux-mm@kvack.org,
        cgroups@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel-team@fb.com
Subject: Re: [PATCH 08/10] psi: pressure stall information for CPU, memory,
 and IO
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On Wed, Jul 18, 2018 at 02:03:18PM +0200, Peter Zijlstra wrote:

> Leaving us just 5 bytes short of needing a single cacheline :/
> 
> struct ponies {
>         unsigned int               tasks[3];                                             /*     0    12 */
>         unsigned int               cpu_state:2;                                          /*    12:30  4 */
>         unsigned int               io_state:2;                                           /*    12:28  4 */
>         unsigned int               mem_state:2;                                          /*    12:26  4 */
> 
>         /* XXX 26 bits hole, try to pack */
> 
>         /* typedef u64 */ long long unsigned int     last_time;                          /*    16     8 */
>         /* typedef u64 */ long long unsigned int     some_time[3];                       /*    24    24 */
>         /* typedef u64 */ long long unsigned int     full_time[2];                       /*    48    16 */
>         /* --- cacheline 1 boundary (64 bytes) --- */
>         /* typedef u64 */ long long unsigned int     nonidle_time;                       /*    64     8 */
> 
>         /* size: 72, cachelines: 2, members: 8 */
>         /* bit holes: 1, sum bit holes: 26 bits */
>         /* last cacheline: 8 bytes */
> };
> 
> ARGGH!

It _might_ be possible to use curr->se.exec_start for last_time if you
very carefully audit and place the hooks. I've not gone through it in
detail, but it might just work.