From: Alex Shi <alex.shi@linaro.org>
To: Michael wang <wangyun@linux.vnet.ibm.com>,
mingo@redhat.com, peterz@infradead.org, tglx@linutronix.de,
daniel.lezcano@linaro.org, vincent.guittot@linaro.org,
morten.rasmussen@arm.com
Cc: linux-kernel@vger.kernel.org, akpm@linux-foundation.org,
fengguang.wu@intel.com, linaro-kernel@lists.linaro.org
Subject: Re: [RFC PATCH] sched: find the latest idle cpu
Date: Wed, 15 Jan 2014 14:45:42 +0800 [thread overview]
Message-ID: <52D62E96.6030301@linaro.org> (raw)
In-Reply-To: <52D61DA0.8050206@linux.vnet.ibm.com>
On 01/15/2014 01:33 PM, Michael wang wrote:
> On 01/15/2014 12:07 PM, Alex Shi wrote:
>> > Currently we just try to find least load cpu. If some cpus idled,
>> > we just pick the first cpu in cpu mask.
>> >
>> > In fact we can get the interrupted idle cpu or the latest idled cpu,
>> > then we may get the benefit from both latency and power.
>> > The selected cpu maybe not the best, since other cpu may be interrupted
>> > during our selecting. But be captious costs too much.
> So the idea here is we want to choose the latest idle cpu if we have
> multiple idle cpu for choosing, correct?
yes.
>
> And I guess that was in order to avoid choosing tickless cpu while there
> are un-tickless idle one, is that right?
no, current logical choice least load cpu no matter if it is idle.
>
> What confused me is, what about those cpu who just going to recover from
> tickless as you mentioned, which means latest idle doesn't mean the best
> choice, or even could be the worst (if just two choice, and the longer
> tickless one is just going to recover while the latest is going to
> tickless).
yes, to save your scenario, we need to know the next timer for idle cpu,
but that is not enough, interrupt is totally unpredictable. So, I'd
rather bear the coarse method now.
>
> So what about just check 'ts->tick_stopped' and record one ticking idle
> cpu? the cost could be lower than time comparison, we could reduce the
> risk may be...(well, not so risky since the logical only works when
> system is relaxing with several cpu idle)
first, nohz full also stop tick. second, tick_stopped can not reflect
the interrupt. when the idle cpu was interrupted, it's waken, then be a
good candidate for task running.
--
Thanks
Alex
next prev parent reply other threads:[~2014-01-15 6:46 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-01-15 4:07 [RFC PATCH] sched: find the latest idle cpu Alex Shi
2014-01-15 4:31 ` Michael wang
2014-01-15 4:48 ` Alex Shi
2014-01-15 4:53 ` Alex Shi
2014-01-15 5:06 ` Alex Shi
2014-01-15 5:33 ` Michael wang
2014-01-15 6:45 ` Alex Shi [this message]
2014-01-15 8:05 ` Michael wang
2014-01-15 14:28 ` Alex Shi
2014-01-15 7:35 ` Peter Zijlstra
2014-01-15 14:37 ` Alex Shi
2014-01-16 11:03 ` Daniel Lezcano
2014-01-16 11:38 ` Peter Zijlstra
2014-01-16 12:16 ` Daniel Lezcano
2014-01-17 2:40 ` Nicolas Pitre
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