Re: [PATCH v1 11/11] locks: give the blocked_hash its own spinlock

From: Jeff Layton
Date: Tue Jun 04 2013 - 10:40:24 EST

On Tue, 04 Jun 2013 16:19:53 +0200
"Stefan (metze) Metzmacher" <metze@xxxxxxxxx> wrote:

> Hi Jeff,
> > There's no reason we have to protect the blocked_hash and file_lock_list
> > with the same spinlock. With the tests I have, breaking it in two gives
> > a barely measurable performance benefit, but it seems reasonable to make
> > this locking as granular as possible.
> as file_lock_{list,lock} is only used for debugging (/proc/locks) after this
> change, I guess it would be possible to use RCU instead of a spinlock.
> @others: this was the related discussion on IRC
> ( about this:
> 16:02 < metze> jlayton: do you have time to discuss your file_lock_lock
> changes?
> 16:02 < jlayton> metze: sure, what's up?
> 16:03 < jlayton> metze: note that it won't help vl's thundering herd
> problems...
> 16:03 < metze> is it correct that after your last patch file_lock_lock
> is only used for /proc/locks?
> 16:03 < jlayton> well, it's only used to protect the list that is used
> for /proc/locks
> 16:04 < jlayton> it still gets taken whenever a lock is acquired or
> released in order to manipulate that list
> 16:04 < metze> would it be a good idea to use rcu instead of a spin lock?
> 16:04 < jlayton> I tried using RCU, but it turned out to slow everything
> down
> 16:04 < jlayton> this is not a read-mostly workload unfortunately
> 16:04 < jlayton> so doing it with mutual exclusion turns out to be faster
> 16:04 < metze> ok
> 16:05 < jlayton> I might play around with it again sometime, but I don't
> think it really helps. What we need to ensure is
> that we optimize the code that manipulates that list,
> and RCU list manipulations have larger overhead
> 16:06 < jlayton> metze: that's a good question though so if you want to
> ask it on the list, please do
> 16:06 < jlayton> others will probably be wondering the same thing
> 16:08 < metze> maybe it's worth a comment in commit message and the code

I'm not sure it's worth commenting about RCU in the code. In the future
it may be possible to do this -- who knows. It does seem to have been
consistently slower in my testing here though.

> 16:08 < metze> btw, why don't you remove the ' /* Protects the
> file_lock_list and the blocked_hash */' comment?

Removed in my git tree -- thanks for pointing that out.

Jeff Layton <jlayton@xxxxxxxxxx>

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