Re: [PATCH tip/locking/core v9 4/6] locking/pvqspinlock: Collect slowpath lock statistics

[Waiman Long]

On 11/05/2015 11:43 AM, Peter Zijlstra wrote:
> On Thu, Nov 05, 2015 at 11:29:29AM -0500, Waiman Long wrote:
> > On 11/02/2015 11:40 AM, Peter Zijlstra wrote:
> > > On Fri, Oct 30, 2015 at 07:26:35PM -0400, Waiman Long wrote:
> > > > This patch enables the accumulation of kicking and waiting related
> > > > PV qspinlock statistics when the new QUEUED_LOCK_STAT configuration
> > > > option is selected. It also enables the collection of data which
> > > > enable us to calculate the kicking and wakeup latencies which have
> > > > a heavy dependency on the CPUs being used.
> > > >
> > > > The statistical counters are per-cpu variables to minimize the
> > > > performance overhead in their updates. These counters are exported
> > > > via the sysfs filesystem under the /sys/kernel/qlockstat directory.
> > > > When the corresponding sysfs files are read, summation and computing
> > > > of the required data are then performed.
> > > Why did you switch to sysfs? You can create custom debugfs files too.
> > I was not aware of that capability. So you mean using debugfs_create_file()
> > using custom file_operations. Right?
> Yep.
>
> > That doesn't seem to be easier than
> > using sysfs. However, I can use that if you think it is better to use
> > debugfs.
> Mostly I just wanted to point out that it was possible; you need not
> change to sysfs because debugfs lacks the capability.
>
> But now that you ask, I think debugfs might be the better place, such
> statistics (and the proposed CONFIG symbol) are purely for debug
> purposes, right?

Davidlohr had asked me to use per-cpu counters to reduce performance 
overhead so that they can be usable in production system. That is 
another reason why I move to sysfs.

BTW, do you have comments on the other patches in the series? I would 
like to collect all the comments before I renew the series.

Cheers,
Longman


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