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

[Jeremy Fitzhardinge <jeremy@goop.org>]

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.

    J