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From: Valentin Schneider <valentin.schneider@arm.com>
To: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: linux-kernel@vger.kernel.org, mingo@kernel.org,
	peterz@infradead.org, vincent.guittot@linaro.org
Subject: Re: [PATCH v2 2/9] sched/debug: Make sd->flags sysctl read-only
Date: Thu, 19 Mar 2020 12:04:56 +0000	[thread overview]
Message-ID: <jhjwo7go8if.mognet@arm.com> (raw)
In-Reply-To: <4a7fe6ae-3587-4a55-1cf2-c4fe568a5ffa@arm.com>


On Thu, Mar 19 2020, Dietmar Eggemann wrote:
> On 11.03.20 19:15, Valentin Schneider wrote:
>> Writing to the sysctl of a sched_domain->flags directly updates the value of
>> the field, and goes nowhere near update_top_cache_domain(). This means that
>> the cached domain pointers can end up containing stale data (e.g. the
>> domain pointed to doesn't have the relevant flag set anymore).
>>
>> Explicit domain walks that check for flags will be affected by
>> the write, but this won't be in sync with the cached pointers which will
>> still point to the domains that were cached at the last sched_domain
>> build.
>>
>> In other words, writing to this interface is playing a dangerous game. It
>> could be made to trigger an update of the cached sched_domain pointers when
>> written to, but this does not seem to be worth the trouble. Make it
>> read-only.
>
> As long as I don't change SD flags for which cached SD pointers exist
> (SD_SHARE_PKG_RESOURCES, SD_NUMA, SD_ASYM_PACKING or
> SD_ASYM_CPUCAPACITY) the write-able interface still could make some sense.
>
> E.g. by enabling SD_BALANCE_WAKE on the fly, I can force !want_affine
> wakees into slow path.
>

True, although since there is no explicit differenciation between the
cached and !cached flags, this is still a landmined interface.

> The question is, do people use the writable flags interface to tweak
> select_task_rq_fair() behavior in this regard?

I did try asking around on IRC (which admittedly is just a small subset
of people) but didn't find anyone.

  reply	other threads:[~2020-03-19 12:05 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-11 18:15 [PATCH v2 0/9] sched: Streamline select_task_rq() & select_task_rq_fair() Valentin Schneider
2020-03-11 18:15 ` [PATCH v2 1/9] sched/fair: find_idlest_group(): Remove unused sd_flag parameter Valentin Schneider
2020-03-19  9:05   ` Dietmar Eggemann
2020-03-11 18:15 ` [PATCH v2 2/9] sched/debug: Make sd->flags sysctl read-only Valentin Schneider
2020-03-19  9:07   ` Dietmar Eggemann
2020-03-19 12:04     ` Valentin Schneider [this message]
2020-03-11 18:15 ` [PATCH v2 3/9] sched: Remove checks against SD_LOAD_BALANCE Valentin Schneider
2020-03-19 10:28   ` Dietmar Eggemann
2020-03-19 12:05     ` Valentin Schneider
2020-03-23 14:26       ` Dietmar Eggemann
2020-03-23 17:17         ` Valentin Schneider
2020-03-11 18:15 ` [PATCH v2 4/9] sched/topology: Kill SD_LOAD_BALANCE Valentin Schneider
2020-03-19 10:29   ` Dietmar Eggemann
2020-03-19 12:06     ` Valentin Schneider
2020-03-11 18:15 ` [PATCH v2 5/9] sched: Add WF_TTWU, WF_EXEC wakeup flags Valentin Schneider
2020-03-11 18:15 ` [PATCH v2 6/9] sched: Kill select_task_rq()'s sd_flag parameter Valentin Schneider
2020-03-11 18:15 ` [PATCH v2 7/9] sched/fair: Dissociate wakeup decisions from SD flag value Valentin Schneider
2020-03-11 18:16 ` [PATCH v2 8/9] sched/fair: Split select_task_rq_fair want_affine logic Valentin Schneider
2020-03-19 10:30   ` Dietmar Eggemann
2020-03-19 12:06     ` Valentin Schneider
2020-03-11 18:16 ` [PATCH v2 9/9] sched/topology: Define and use shortcut pointers for wakeup sd_flag scan Valentin Schneider
2020-03-19 10:46   ` Dietmar Eggemann
2020-03-19 12:22     ` Valentin Schneider

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