From: Valentin Schneider <valentin.schneider@arm.com>
To: Quentin Perret <qperret@google.com>
Cc: linux-kernel@vger.kernel.org, mingo@kernel.org,
peterz@infradead.org, vincent.guittot@linaro.org,
dietmar.eggemann@arm.com, morten.rasmussen@arm.com
Subject: Re: [PATCH v3 2/7] sched/topology: Define and assign sched_domain flag metadata
Date: Thu, 02 Jul 2020 17:49:38 +0100 [thread overview]
Message-ID: <jhjeept980t.mognet@arm.com> (raw)
In-Reply-To: <20200702163748.GA1125675@google.com>
On 02/07/20 17:37, Quentin Perret wrote:
> On Thursday 02 Jul 2020 at 17:25:41 (+0100), Valentin Schneider wrote:
>> It's actually pretty close to what happens with the LLC domain on SMP -
>> select_idle_sibling() doesn't look outside of it. The wake_affine() stuff
>> might steer the task towards a different LLC, but that's about it for
>> wakeups. We rely on load balancing (fork/exec, newidle, nohz and periodic)
>> to spread this further - and we would here too.
>
> Sure, but on SMP the search space in select_idle_sibling is always
> consistent -- you search within the LLC. With the fix you suggested,
> CPUs 0-3 will search within their LLCs, while CPU4 searches the entire
> system, which creates an imbalanced mess IMO.
>
Yeah, it is a mess.
> For affine wake-ups, you could migrate from CPU4 -> CPU0-3, but CPU0-3
> to CPU4 is not possible
AIU the wake_affine bits, you get to steer the wakeup towards the waking
CPU. So if the task previously ran on CPU0-3, wake_affine can make the
target CPU4 (waking CPU), so it would become a possible candidate.
But as you say, this thing is still an ugly asymmetric mess.
> so this asymmetry is almost guaranteed to
> actively create imbalance. And sure, the periodic load balancer ought to
> fix it, but really wake-up balance and periodic load balance should be
> pushing in the same direction and not fighting against each other.
>
> Anyways, enough bikeshedding for today, I'll try and have look at the
> rest of the series :)
>
Thanks!
> Cheers,
> Quentin
next prev parent reply other threads:[~2020-07-02 16:49 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-01 19:06 [PATCH v3 0/7] sched: Instrument sched domain flags Valentin Schneider
2020-07-01 19:06 ` [PATCH v3 1/7] sched/topology: Split out SD_* flags declaration to its own file Valentin Schneider
2020-07-01 19:06 ` [PATCH v3 2/7] sched/topology: Define and assign sched_domain flag metadata Valentin Schneider
2020-07-02 12:15 ` Quentin Perret
2020-07-02 14:31 ` Valentin Schneider
2020-07-02 15:45 ` Quentin Perret
2020-07-02 16:25 ` Valentin Schneider
2020-07-02 16:37 ` Quentin Perret
2020-07-02 16:49 ` Valentin Schneider [this message]
2020-07-01 19:06 ` [PATCH v3 3/7] sched/topology: Verify SD_* flags setup when sched_debug is on Valentin Schneider
2020-07-02 14:20 ` Peter Zijlstra
2020-07-02 14:32 ` Valentin Schneider
2020-07-01 19:06 ` [PATCH v3 4/7] arm, sched/topology: Remove SD_SHARE_POWERDOMAIN Valentin Schneider
2020-07-02 16:44 ` Dietmar Eggemann
2020-07-02 18:46 ` Valentin Schneider
2020-07-01 19:06 ` [PATCH v3 5/7] sched/topology: Add more flags to the SD degeneration mask Valentin Schneider
2020-07-02 18:28 ` Dietmar Eggemann
2020-07-01 19:06 ` [PATCH v3 6/7] sched/topology: Introduce SD metaflag for flags needing > 1 groups Valentin Schneider
2020-07-02 18:29 ` Dietmar Eggemann
2020-07-02 18:46 ` Valentin Schneider
2020-07-13 12:39 ` Peter Zijlstra
2020-07-13 13:25 ` Valentin Schneider
2020-07-01 19:06 ` [PATCH v3 7/7] sched/topology: Use prebuilt SD flag degeneration mask Valentin Schneider
2020-07-13 12:55 ` Peter Zijlstra
2020-07-13 13:28 ` Valentin Schneider
2020-07-13 13:43 ` Peter Zijlstra
2020-07-13 13:52 ` Valentin Schneider
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