Re: cpufreq cleanups - .30 vs .31

Mathieu Desnoyers
2009-07-06T13:45:57Z

* Thomas Renninger (trenn@suse.de) wrote:
> Hi Dave,
>
> this is about Venki's and Mathieu's recently sent cleanups.
> I'd like to summarize this to help finding a solution:
>
> IMO Venki's approach (making .governor() always be called with
> rwsem held) is the cleaner one and this should be the way to
> go for .31 and future. This better separates locking responsibilities
> between cpufreq core and governors and brings back "design" into this.
>
> One could argue that for .30 Mathieu's is better, because less
> intrusive. It's up to Dave in the end, but:
> [patch 2.6.30 1/4] remove rwsem lock from CPUFREQ_GOV_STOP call (second call
> site)
> should not be the way to go for .31 and I'd vote for Venki's
> approach concerning locking .governor() against multiple calls (done by
> rwsem) and governor() vs do_dbs_timer calls (governor's job with a governor
> specific sem).
>
> So if not find too intrusive, I'd say:
> Venkatesh's whole series of:
> [patch 0/4] Take care of cpufreq lockdep issues (take 2)
> should be seen in .31.
>
> Depending on how intrusive this is seen, Venki's first patch:
> [patch 1/4] cpufreq: Eliminate the recent lockdep warnings in cpufreq
> should then go to .30 (after still waiting a bit?)
> or Mathieu's approach (I'd vote for Venki's to be consistent for .30 and .31).
>
> The one patch from Mathieu:
> [patch 2.6.30 2/4] CPUFREQ: fix (utter) cpufreq_add_dev mess
> is a separate, general cleanup which should show up in .31.
>
>
>
> I still have two patch specific questions:
> about Mathieu's (it's a minor issue in the error path):
> [patch 2.6.30 2/4] CPUFREQ: fix (utter) cpufreq_add_dev mess
>
> +                       if (lock_policy_rwsem_write(cpu) < 0) {
> +                               /* Should not go through policy unlock path */
> +                               if (cpufreq_driver->exit)
> +                                       cpufreq_driver->exit(policy);
> +                               ret = -EBUSY;
> +                               cpufreq_cpu_put(managed_policy);
> Shouldn't:
> cpufreq_cpu_put(managed_policy);
> be called before:
> cpufreq_driver->exit(policy);
> Just in case the driver itself wants to grab the policy of the
> managed cpu?
>

If we want to make it perfectly similar to the error path:

(note : this is a success path if ret = 0, and error path if ret != 0)

ret != 0 :
if (!ret)
cpufreq_cpu_put(managed_policy);
/*
* Success. We only needed to be added to the mask.
* Call driver->exit() because only the cpu parent of
* the kobj needed to call init().
*/
goto out_driver_exit; /* call driver->exit() */

Then yes, we might be tempted to flip the cpu_put and exit.

But given cpu_put just decrements a reference counter (and performs
cleanup if needed), then even if ->exit() takes a reference count
somehow, this will just make the code hold 2 refcounts temporarily.

And I prefer to keep this refcount thorough ->exit() call, because we
don't hold any policy_rwsem at this point : we are in a lock acquisition
error path. It's therefore safer to keep the refcount to ensure that
data won't vanish.

Mathieu

>
> about Venki's:
> [patch 3/4] cpufreq: Cleanup locking in ondemand governor
> Isn't it possible to use only one mutex(timer_mutex) to protect do_dbs_timer
> against governor start, stop, limit?
> Then dbs_mutex would only be used to protect against concurrent sysfs access
> and can be thrown away as soon as ondemand only provides global sysfs files,
> not per cpu ones.
>
> Hmm, maybe this should just go in? It eases up things, but it's still hard
> to follow up each detail. Fixes/enhancements can still be put on top
> for .31.
>
> Thomas
>

--
Mathieu Desnoyers
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