Re: [PATCH 2/2] perf lock: New subcommand "lock" to perf for analyzing lock statistics

[Frederic Weisbecker <fweisbec@gmail.com>]

On Mon, Dec 07, 2009 at 12:34:44PM +0900, Hitoshi Mitake wrote:
> This patch adds new subcommand "lock" to perf for analyzing lock usage statistics.
> Current perf lock is very primitive. This cannot provide the minimum functions.
> Of course I continue to working on this.
> But too big patch is not good thing for you, so I post this.


Oh great!
Yeah, the work can be done incrementally.


> 
> 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.



> 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?


> 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

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.


> 
> 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.


> 
> 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.