linux-kernel - Re: [patch] sched: implement cpu_clock(cpu) high-speed time source, take #2

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Message-ID: <20070719182007.GB11657@kernel.dk>
Date:	Thu, 19 Jul 2007 20:20:07 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	stable@...nel.org, Greg KH <greg@...ah.com>,
	Chris Wright <chrisw@...s-sol.org>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	Jan Glauber <jang@...ux.vnet.ibm.com>, Andi Kleen <ak@...e.de>
Subject: Re: [patch] sched: implement cpu_clock(cpu) high-speed time
	source,   take #2

On Thu, Jul 19 2007, Jeremy Fitzhardinge wrote:
> Jens Axboe wrote:
> > On Thu, Jul 19 2007, Jeremy Fitzhardinge wrote:
> >   
> >>> Index: linux/block/blktrace.c
> >>> ===================================================================
> >>> --- linux.orig/block/blktrace.c
> >>> +++ linux/block/blktrace.c
> >>> @@ -41,7 +41,7 @@ static void trace_note(struct blk_trace 
> >>>  		const int cpu = smp_processor_id();
> >>>  
> >>>  		t->magic = BLK_IO_TRACE_MAGIC | BLK_IO_TRACE_VERSION;
> >>> -		t->time = sched_clock() - per_cpu(blk_trace_cpu_offset, cpu);
> >>> +		t->time = cpu_clock(cpu) - per_cpu(blk_trace_cpu_offset, cpu);
> >>>  		t->device = bt->dev;
> >>>  		t->action = action;
> >>>  		t->pid = pid;
> >>> @@ -159,7 +159,7 @@ void __blk_add_trace(struct blk_trace *b
> >>>  
> >>>  		t->magic = BLK_IO_TRACE_MAGIC | BLK_IO_TRACE_VERSION;
> >>>  		t->sequence = ++(*sequence);
> >>> -		t->time = sched_clock() - per_cpu(blk_trace_cpu_offset, cpu);
> >>> +		t->time = cpu_clock(cpu) - per_cpu(blk_trace_cpu_offset, cpu);
> >>>   
> >>>       
> >> What's this measuring here?  Time spend in IO?  Wouldn't it be better
> >> off with a measurement of real monotonic time?
> >>     
> >
> > It's not time spent in IO, it wants a nanosecond timestamp.
> >   
> 
> What kind of nanoseconds?  Real?  Virtual?
> 
> sched_clock() (and now cpu_clock()) measure the number of nanoseconds
> available for running CPU instructions (regardless of whether it chooses
> to use them or not), and so in a virtual environment doesn't include
> time stolen by the hypervisor for other VCPUs.

Real time, it's a timestamp that the user tools can use to see the time
delta between A and B.

So I guess blktrace currently doesn't work so well inside a gues...

-- 
Jens Axboe

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