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From:   Dietmar Eggemann <dietmar.eggemann@arm.com>
Subject: Re: [PATCH v2 2/9] sched/debug: Make sd->flags sysctl read-only
To:     Valentin Schneider <valentin.schneider@arm.com>,
        linux-kernel@vger.kernel.org
Cc:     mingo@kernel.org, peterz@infradead.org, vincent.guittot@linaro.org
References: <20200311181601.18314-1-valentin.schneider@arm.com>
 <20200311181601.18314-3-valentin.schneider@arm.com>
Message-ID: <4a7fe6ae-3587-4a55-1cf2-c4fe568a5ffa@arm.com>
Date:   Thu, 19 Mar 2020 10:07:21 +0100
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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.

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