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From: Valentin Schneider <valentin.schneider@arm.com>
To: Steven Sistare <steven.sistare@oracle.com>,
	mingo@redhat.com, peterz@infradead.org
Cc: subhra.mazumdar@oracle.com, dhaval.giani@oracle.com,
	daniel.m.jordan@oracle.com, pavel.tatashin@microsoft.com,
	matt@codeblueprint.co.uk, umgwanakikbuti@gmail.com,
	riel@redhat.com, jbacik@fb.com, juri.lelli@redhat.com,
	vincent.guittot@linaro.org, quentin.perret@arm.com,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 03/10] sched/topology: Provide cfs_overload_cpus bitmap
Date: Thu, 6 Dec 2018 17:28:37 +0000	[thread overview]
Message-ID: <c872ddfb-eb12-e293-7ade-ef602ac45b1c@arm.com> (raw)
In-Reply-To: <2e1f4d52-8ac6-04a3-c453-1679ef3df0e7@oracle.com>

Hi Steve,

On 06/12/2018 16:40, Steven Sistare wrote:
> [...]
>>
>> Ah yes, that would work. Thing is, I had excluded having the misfit masks
>> being in the sd_llc_shareds, since from a logical standpoint they don't
>> really belong there.
>>
>> With asymmetric CPU capacities we kind of disregard the cache landscape
> 
> Sure, but adding awareness of the cache hierarchy can only make it better,
> and a per-LLC mask organization can serve both the overloaded and misfit
> use cases quite naturally.
> [...]
>> So in truth I was envisioning separate SD_ASYM_CPUCAPACITY-based 
>> sparsemasks, which is why I was rambling about SD_ASYM_CPUCAPACITY siblings
>> of sd_llc_*()... *But* after I had a go at it, it looked to me like that
>> was a lot of duplicated code.
> 
> I would be happy to review your code and make suggestions to reduce duplication,
> and happy to continue to discuss clean and optimal handling for misfits. However, 
> I have a request: can we push my patches across the finish line first?  Stealing 
> for misfits can be its own patch series.  Please consider sending your reviewed-by
> for the next version of my series.  I will send it soon.
> 

Sure, as things stand right now I'm fairly convinced this doesn't harm
asymmetric systems.

The only thing I would add (ignoring misfits) is that with EAS we would
need to gate stealing with something like:

    !static_branch_unlikely(&sched_energy_present) ||
    READ_ONCE(rq->rd->overutilized)

And who "gets" to add this gating (or at least, when must it be added)
depends on which patch-set gets in first.

[...]
>> Sadly I think that doesn't work as well for cfs_overload_cpus since you
>> can't split a sparsemask's chunks over several NUMA nodes, so we'd be
>> stuck with an allocation on a single node (but we already do that in some
>> places, e.g. for nohz.idle_cpus_mask, so... Is it that bad?).
> 
> It can be bad for high memory bandwidth workloads, as the sparsemasks will
> be displaced from cache and we incur remote memory latencies on next access.
> 

Aye, I just caught up with the LPC videos and was about to reply here to
say that, all things considered, it's probably not such a good idea...

> - Steve
> 

  reply	other threads:[~2018-12-06 17:28 UTC|newest]

Thread overview: 30+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-11-09 12:50 [PATCH v3 00/10] steal tasks to improve CPU utilization Steve Sistare
2018-11-09 12:50 ` [PATCH v3 01/10] sched: Provide sparsemask, a reduced contention bitmap Steve Sistare
2018-11-27 15:16   ` Steven Sistare
2018-11-28  1:19     ` Omar Sandoval
2018-12-06 16:07       ` Steven Sistare
2018-12-06 18:19         ` Omar Sandoval
2018-11-09 12:50 ` [PATCH v3 02/10] sched/topology: Provide hooks to allocate data shared per LLC Steve Sistare
2018-11-09 12:50 ` [PATCH v3 03/10] sched/topology: Provide cfs_overload_cpus bitmap Steve Sistare
2018-11-09 17:38   ` Valentin Schneider
2018-11-19 17:32     ` Steven Sistare
2018-11-20 12:52       ` Valentin Schneider
2018-11-12 16:42   ` Valentin Schneider
2018-11-19 17:33     ` Steven Sistare
2018-11-20 12:42       ` Valentin Schneider
2018-11-26 19:06         ` Steven Sistare
2018-12-03 16:56           ` Valentin Schneider
2018-12-06 16:40             ` Steven Sistare
2018-12-06 17:28               ` Valentin Schneider [this message]
2018-11-09 12:50 ` [PATCH v3 04/10] sched/fair: Dynamically update cfs_overload_cpus Steve Sistare
2018-11-09 12:50 ` [PATCH v3 05/10] sched/fair: Hoist idle_stamp up from idle_balance Steve Sistare
2018-11-09 19:07   ` Valentin Schneider
2018-11-19 17:31     ` Steven Sistare
2018-11-20 10:24       ` Valentin Schneider
2018-11-09 12:50 ` [PATCH v3 06/10] sched/fair: Generalize the detach_task interface Steve Sistare
2018-11-09 12:50 ` [PATCH v3 07/10] sched/fair: Provide can_migrate_task_llc Steve Sistare
2018-11-09 12:50 ` [PATCH v3 08/10] sched/fair: Steal work from an overloaded CPU when CPU goes idle Steve Sistare
2018-11-09 12:50 ` [PATCH v3 09/10] sched/fair: disable stealing if too many NUMA nodes Steve Sistare
2018-11-09 12:50 ` [PATCH v3 10/10] sched/fair: Provide idle search schedstats Steve Sistare
2018-11-10 17:08   ` kbuild test robot
2018-11-09 15:02 ` hackbench run scripts Steven Sistare

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