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SubjectRe: [PATCH 2/2] perf lock: New subcommand "lock" to perf for analyzing lock statistics
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From: Frederic Weisbecker <fweisbec@gmail.com>
Subject: Re: [PATCH 2/2] perf lock: New subcommand "lock" to perf for analyzing lock statistics
Date: Mon, 7 Dec 2009 05:41:26 +0100

Frederic, thanks for your comment!

> >
> > And I found some important problem, so I'd like to ask your opinion.
> > For another issue, this patch depends on the previous one.
> > The previous one is very dirty and temporary, I cannot sign on it, so I cannot sign on this too...
>
>
>
> The previous one looks rather good actually.

Thanks for your review in previous mail.
I'm new to perf, so I didn't have confidence.
Your advice is encouraging!

>
>
>
> > First, it seems that current locks (spinlock, rwlock, mutex) has no numeric ID.
> > So we can't treat rq->lock on CPU 0 and rq->lock on CPU 1 as different things.
> > Symbol name of locks cannot be complete ID.
> > This is the result of current ugly data structure for lock_stat
> > (data structure for stat per lock in builtin-lock.c).
> > Hash table will solve the problem of speed,
> > but it is not a radical solution.
> > I understand it is hard to implement numeric IDs for locks,
> > but it is required seriously, do you have some ideas?
>
>
> Indeed. I think every lock instance has its own lockdep_map.
> And this lockdep_map is passed in every lock event but is
> only used to retrieve the name of the lock.
>
> Why not adding the address of the lockdep_map in the event?

It's good idea. Address cannot be used as index of array directly,
but dealing with it is far easier than string and low cost.

>
>
> > Second, there's a lot of lack of information from trace events.
> > For example, current lock event subsystem cannot provide the time between
> > lock_acquired and lock_release.
> > But this time is already measured in lockdep, and we can obtain it
> > from /proc/lock_stat.
> > But /proc/lock_stat provides information from boot time only.
> > So I have to modify wide area of kernel including lockdep, may I do this?
>
>
>
> I think this is more something to compute in a state machine:
> lock_release - lock_acquired event.
>
> This is what we do with sched events in perf sched latency

Yeah, tracing state of the lock is smart way. I'll try it.

>
> Also I think we should remove the field that gives the time waited
> between lock_acquire and lock_acquired. This is more something that
> should be done in userspace instead of calculating in from the kernel.
> This brings overhead in the wrong place.

I agree. I think we can exploit more information from timestamps.

>
>
> >
> > Third, siginificant overhead :-(
> >
> > % perf bench sched messaging # Without perf lock rec
> >
> > Total time: 0.436 [sec]
> >
> > % sudo ./perf lock rec perf bench sched messaging # With perf lock rec
> >
> > Total time: 4.677 [sec]
> > [ perf record: Woken up 0 times to write data ]
> > [ perf record: Captured and wrote 106.065 MB perf.data (~4634063 samples) ]
> >
> > Over 10 times! No one can ignore this...
>
>
> I think that the lock events are much more sensible than the sched events,
> and that by nature: these are very high frequency events class, probably the
> highest among every event classes we have (the worst beeing function tracing :)
>
> But still, you're right, there are certainly various things we need to
> optimize in this area.
>
> More than 8 times slower is high.

It seems that lockdep contains some O(n) codes.
Of course lockdep is important, but analyzing statistics of lock usage
is another problem.
I think separating lockdep and lock event for stats can be solution.

>
>
> >
> > This is example of using perf lock prof:
> > % sudo ./perf lock prof # Outputs in pager
> > ------------------------------------------------------------------------------------------
> > Lock | Acquired | Max wait ns | Min wait ns | Total wait ns |
> > --------------------------------------------------------------------------------------------
> > &q->lock 30 0 0 0
> > &ctx->lock 3912 0 0 0
> > event_mutex 2 0 0 0
> > &newf->file_lock 1008 0 0 0
> > dcache_lock 444 0 0 0
> > &dentry->d_lock 1164 0 0 0
> > &ctx->mutex 2 0 0 0
> > &child->perf_event_mutex 2 0 0 0
> > &event->child_mutex 18 0 0 0
> > &f->f_lock 2 0 0 0
> > &event->mmap_mutex 2 0 0 0
> > &sb->s_type->i_mutex_key 259 0 0 0
> > &sem->wait_lock 27205 0 0 0
> > &(&ip->i_iolock)->mr_lock 130 0 0 0
> > &(&ip->i_lock)->mr_lock 6376 0 0 0
> > &parent->list_lock 9149 7367 146 527013
> > &inode->i_data.tree_lock 12175 0 0 0
> > &inode->i_data.private_lock 6097 0 0 0
>
>
>
> Very nice and promising!
>
> I can't wait to try it.
>
>

Thanks! I'll do my best :)


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