linux-kernel - Re: [PATCH] kvm: add halt_poll

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Message-ID: <20150209172818.GB1804@potion.brq.redhat.com>
Date:	Mon, 9 Feb 2015 18:28:19 +0100
From:	Radim Krčmář <rkrcmar@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org, riel@...hat.com,
	mtosatti@...hat.com, jan.kiszka@...mens.com, dmatlack@...gle.com
Subject: Re: [PATCH] kvm: add halt_poll_ns module parameter

2015-02-09 17:10+0100, Paolo Bonzini:
> On 09/02/2015 16:21, Radim Krčmář wrote:
> > 2015-02-06 13:48+0100, Paolo Bonzini:
> >> +TRACE_EVENT(kvm_vcpu_wakeup,
> >> +	    TP_PROTO(__u64 ns, bool waited),
> > 
> > (__u64 is preferred here?)
> 
> Preferred to what?

To 'u64'.  (The header file shouldn't be reachable from user-space.)

> >> +		} while (single_task_running() && ktime_before(cur, stop));
> > 
> > After reading a bunch of code, I'm still not sure ...
> >  - need_resched() can't be true when single_task_running()?
> >    (I think it could happen -- balancing comes into mind.)
> 
> Single_task_running is per-CPU; for a task to relinquish control to
> another task, you first need to have multiple tasks running.  In other
> words, I think it cannot.

Ok, thanks.  (I thought that need_resched has more general meaning and
couldn't confirm that balancing/CPU-offlining/... just evicts the task
without waiting for its schedule().)
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