Re: [PATCH] procfs: Improve Scaling in proc

[Nathan Zimmer]

On 10/18/2012 02:46 AM, Eric Dumazet wrote:
> On Wed, 2012-10-17 at 15:25 -0500, Nathan Zimmer wrote:
> > I am currently tracking a hotlock reported by a customer on a large, 512 cores,
> > system, I am currently running 3.7.0 rc1 but the issue looks like it has been
> > this way for a very long time.
> > The offending lock is proc_dir_entry->pde_unload_lock.
> >
> > This patch converts the replaces the lock with the rcu. It is a refresh of what
> > was orignally suggested by Eric Dumazet.  I refreshed it to the 3.7.
> >
> > Supporting numbers, lower is better, they are from the test I posted earlier.
> > cpuinfo baseline        Rcu
> > tasks   read-sec        read-sec
> > 1       0.0141          0.0141
> > 2       0.0140          0.0142
> > 4       0.0140          0.0141
> > 8       0.0145          0.0140
> > 16      0.0553          0.0168
> > 32      0.1688          0.0549
> > 64      0.5017          0.1690
> > 128     1.7005          0.5038
> > 256     5.2513          2.0804
> > 512     8.0529          3.0162
> >
> >
> > Cc: Eric Dumazet <eric.dumazet@xxxxxxxxx>
> > Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
> > Cc: David Woodhouse <dwmw2@xxxxxxxxxxxxx>
> > Cc: Alexey Dobriyan <adobriyan@xxxxxxxxx>
> > Signed-off-by: Nathan Zimmer <nzimmer@xxxxxxx>
>
> Hmm, this patch had several issues and I had no time yet to work on a
> new version. I probably wont have time in a near future.
>
> Paul sent me some comments about it, I hope he doesnt mind I copy them
> here, if you want to polish the patch.
>
> Thanks !

I'll try to polish this up and resend it.
And any comments are most welcome.


> On Wed, 2012-10-03 at 10:56 -0700, Paul E. McKenney wrote:
> > Finally getting back to this...  :-/
> >
> > Why not set the initial value of the reference counter to 1
> > (rather than zero), continue acquiring with atomic_inc(), but
> > use atomic_dec_and_test() to decrement?  Put a completion in
> > the data structure, so if the atomic_dec_and_test() indicates that
> > the counter is now zero, do a complete().
> >
> > Then to free the object, remove it from the data structure, do a
> > synchronize_rcu(), do an atomic_dec_and_test() to remove the initial
> > value, again doing a complete() if the counter is now zero.  The do
> > a wait_for_completion().
> >
> > This would get rid of the polling loop.
> >
> > So, what am I missing here?  ;-)
> >
> > 							Thanx, Paul

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