Date: Thu, 07 Jun 2012 22:13:30 +0300 From: Alexander Motin <mav@FreeBSD.org> To: Andriy Gapon <avg@FreeBSD.org> Cc: freebsd-acpi@FreeBSD.org Subject: Re: [stable 9] broken hwpstate calls Message-ID: <4FD0FD5A.9070807@FreeBSD.org> In-Reply-To: <4FD0ED44.9040402@FreeBSD.org> References: <1337319129.2915.4.camel@powernoodle-l7> <4FB6765A.2050307@FreeBSD.org> <1337710214.2916.8.camel@powernoodle-l7.corp.yahoo.com> <20120525163653.b61a08e2.lists@yamagi.org> <4FBFA9A9.7020806@FreeBSD.org> <4FBFBD39.7000105@FreeBSD.org> <4FBFDFFB.9020501@FreeBSD.org> <4FBFE624.1020208@FreeBSD.org> <20120526090233.f638c1d2.lists@yamagi.org> <4FC0A3A1.80200@FreeBSD.org> <4FC7D464.20602@FreeBSD.org> <4FCFD2A1.60706@FreeBSD.org> <4FCFE178.9080505@FreeBSD.org> <4FD061F8.6030905@FreeBSD.org> <4FD06885.4050507@FreeBSD.org> <4FD0ED44.9040402@FreeBSD.org>
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On 06/07/12 21:04, Andriy Gapon wrote: > on 07/06/2012 11:38 Alexander Motin said the following: >> On 06/07/12 11:10, Andriy Gapon wrote: >>> on 07/06/2012 02:02 Jung-uk Kim said the following: >>>> Any way, hwpstate still isn't quite right even without your patch. >>>> >>>> sys/kern/kern_cpu.c cpufreq_curr_sysctl() -> CPUFREQ_SET() -> /* for all >>>> CPU devices */ cf_set_method() -> /* thread_lock(), sched_bind(), ... */ >>>> CPUFREQ_DRV_SET() -> sys/x86/cpufreq/hwpstate.c hwpstate_set() -> >>>> hwpstate_goto_pstate() /* for each CPU unit */ /* thread_lock(), >>>> sched_bind(), ... */ >>> >>> Oh, I didn't realize that there was the cpufreq-level loop over all CPUs! >>> That really sucks. >>> >>> Maybe some day we should accept that different CPUs could legitimately be in >>> different P-states and provide support for that throughout the stack (from >>> powerd to drivers). >> >> Support for different P-states on different CPUs can be useful if CPUs have >> different capabilities. > > Not sure what you mean... I was talking about setting different CPUs to > different P-states based on the per-CPU conditions (e.g. utilization). I > certainly didn't mean to talk about heterogeneous P-state definitions or any > other heterogeneous silicon issues. As you wish, but at this moment it is the only realistic application I see. As I've told below, setting different frequencies to different cores without scheduler awareness is a bad idea. >> I believe it is very rare, but possible. At this moment >> cpufreq should set for each CPU frequency closest to one that was set on BSP. It >> should be possible to make powerd to read sets of frequencies from all CPUs and >> do the same, just more intelligently. >> >> Same time using very different frequencies for different CPUs can IMHO be very >> problematic even in theory. For SMP systems it is quite difficult (because of >> threads migration and possible inter-operations of multiple threads) to identify >> cases when even global frequency can be reduced without proportional performance >> penalty. Making in per-CPU multiplies number of options and requires awareness >> from the scheduler. > > I humbly disagree. I think that it's not a job of scheduler to be overly smart > when power-saving policies are in effect. IMO, scheduler should just do its own > job and powerd should react to individual loads of CPUs. Where latencies really > matter there powerd should not be used (or perhaps used with some different > policy skewed towards performance vs economy). Scheduler usually operates in terms of milliseconds or less. powerd operates in best case in terms of fractions of seconds (or it will eat more power then save). Unless you are doing some heavy CPU-bound math without any context switches, it won't work well without scheduler aware about available computation resources. > Also, Linux does it, so it must at least doable :-) I don't know whether or how Linux does it. If you know how to do it effectively -- welcome, be my guest. :) -- Alexander Motin
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