Re: [PATCH] Cpuset: rcu optimization of page alloc hook

[Paul Jackson <pj@sgi.com>]

pj wrote:
> Time for me to learn more about rcu.

Well, that was easy.

Directly using RCU to guard that task->cpuset pointer instead of
cheating via the RCU hooks in the slab cache was just a few lines of
code.

But, boy oh boy, that synchronize_rcu() call sure takes it time.

My cpuset torture test was creating, destroying and abusing about 2600
cpusets/sec before this change, and now it does about 144 cpusets/sec.

That cost 95% of the performance.  This only hits on the cost of
attaching a task to a different cpuset (by writing its <pid> to
some other cpuset 'tasks' file.)

Just commenting out the synchronize_cpu() restores the previous speed.

Nothing surprising here - this is just how rcu claims it behaves,
heavily favoring the read side performance.  These delays are waiting
for the other cpus doing rcu_read_lock() to go through a quite period
(reschedule, idle or return to user code).

Patches coming soon, to remove the cpuset_cache slab cache, and to
use RCU directly instead.

Thanks, Eric.

-- 
                  I won't rest till it's the best ...
                  Programmer, Linux Scalability
                  Paul Jackson <pj@sgi.com> 1.925.600.0401