linux-kernel - Re: cputime takes cstate into consideration

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Message-ID: <20190626101320.GX3402@hirez.programming.kicks-ass.net>
Date:   Wed, 26 Jun 2019 12:13:20 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krcmar <rkrcmar@...hat.com>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        KarimAllah <karahmed@...zon.de>,
        LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>
Subject: Re: cputime takes cstate into consideration

On Wed, Jun 26, 2019 at 05:43:55PM +0800, Wanpeng Li wrote:
> Hi all,
> 
> After exposing mwait/monitor into kvm guest, the guest can make
> physical cpu enter deeper cstate through mwait instruction, however,
> the top command on host still observe 100% cpu utilization since qemu
> process is running even though guest who has the power management
> capability executes mwait. Actually we can observe the physical cpu
> has already enter deeper cstate by powertop on host. Could we take
> cstate into consideration when accounting cputime etc?

Either we account runtime on the CPU itself, in which case it will not
be in a C state due to actually running an interrupt that does
accounting, or we do it remote (NOHZ_FULL case) and there is no way to
know what C state, if any, that CPU is in.