linux-kernel - Re: [RFC][PATCH 2/2] cpufreq: schedutil: Force max frequency on busy CPUs

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <11131190.KAQLyFuH4P@aspire.rjw.lan>
Date:   Mon, 20 Mar 2017 14:05:24 +0100
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Juri Lelli <juri.lelli@....com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Joel Fernandes <joelaf@...gle.com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [RFC][PATCH 2/2] cpufreq: schedutil: Force max frequency on busy CPUs

On Monday, March 20, 2017 01:06:15 PM Patrick Bellasi wrote:
> On 20-Mar 13:50, Peter Zijlstra wrote:
> > On Mon, Mar 20, 2017 at 01:35:12PM +0100, Rafael J. Wysocki wrote:
> > > On Monday, March 20, 2017 11:36:45 AM Peter Zijlstra wrote:
> > > > On Sun, Mar 19, 2017 at 02:34:32PM +0100, Rafael J. Wysocki wrote:
> > > > > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > > > > 
> > > > > The PELT metric used by the schedutil governor underestimates the
> > > > > CPU utilization in some cases.  The reason for that may be time spent
> > > > > in interrupt handlers and similar which is not accounted for by PELT.
> > > > > 
> > > > > That can be easily demonstrated by running kernel compilation on
> > > > > a Sandy Bridge Intel processor, running turbostat in parallel with
> > > > > it and looking at the values written to the MSR_IA32_PERF_CTL
> > > > > register.  Namely, the expected result would be that when all CPUs
> > > > > were 100% busy, all of them would be requested to run in the maximum
> > > > > P-state, but observation shows that this clearly isn't the case.
> > > > > The CPUs run in the maximum P-state for a while and then are
> > > > > requested to run slower and go back to the maximum P-state after
> > > > > a while again.  That causes the actual frequency of the processor to
> > > > > visibly oscillate below the sustainable maximum in a jittery fashion
> > > > > which clearly is not desirable.
> > > > > 
> > > > > To work around this issue use the observation that, from the
> > > > > schedutil governor's perspective, CPUs that are never idle should
> > > > > always run at the maximum frequency and make that happen.
> > > > > 
> > > > > To that end, add a counter of idle calls to struct sugov_cpu and
> > > > > modify cpuidle_idle_call() to increment that counter every time it
> > > > > is about to put the given CPU into an idle state.  Next, make the
> > > > > schedutil governor look at that counter for the current CPU every
> > > > > time before it is about to start heavy computations.  If the counter
> > > > > has not changed for over SUGOV_BUSY_THRESHOLD time (equal to 50 ms),
> > > > > the CPU has not been idle for at least that long and the governor
> > > > > will choose the maximum frequency for it without looking at the PELT
> > > > > metric at all.
> > > > 
> > > > Why the time limit?
> > > 
> > > One iteration appeared to be a bit too aggressive, but honestly I think
> > > I need to check again if this thing is regarded as viable at all.
> > > 
> > 
> > I don't hate the idea; if we don't hit idle; we shouldn't shift down. I
> > just wonder if we don't already keep a idle-seqcount somewhere; NOHZ and
> > RCU come to mind as things that might already use something like that.
> 
> Maybe the problem is not going down (e.g. when there are only small
> CFS tasks it makes perfectly sense) but instead not being fast enough
> on rampin-up when a new RT task is activated.
> 
> And this boils down to two main point:
> 1) throttling for up transitions perhaps is only harmful
> 2) the call sites for schedutils updates are not properly positioned
>    in specific scheduler decision points.
> 
> The proposed patch is adding yet another throttling mechanism, perhaps
> on top of one which already needs to be improved.

It is not throttling anything.

Thanks,
Rafael