Re: 2.6.14-rc4 latency issue with rcu_process_callbacks()/file_free_rcu()

[Eric Dumazet]

Dimitri Sivanich a écrit :
> Just bringing up a latency issue I've noticed recently.
>
> In or around 2.6.14-rc4 some changes were made to have the call to
> kmem_cache_free() from file_free() in the Linux kernel be deferred, running
> as a tasklet via file_free_rcu(), rather than running kmem_cache_free()
> right from file_free() directly.
>
> I've noticed that rcu_process_callbacks() can take quite a while to run
> now that it routinely calls file_free_rcu() to run kmem_cache_free().
> This can make the cpu unavailable for 100's of usec on 1GHz machines, with
> or without preemption configured on (much of this path is non-preemptible).
>
> This can result in some unpredictable periods of fairly long cpu latency,
> such as when a thread is waiting to be woken by an interrupt handler on a
> 'now quiet' cpu.  Changing file_free() to call kmem_cache_free() directly
> completely eliminates this unexpected latency.

Well, you cannot change file_free() to call kmem_cache_free() directly, or 
risk corruption/crash.

See Documentation/RCU/UP.txt

Dont you notice latency issue with other RCU protected data, like dentries ?

BTW a change in 2.6.14-rc5 might give different latency results.

Eric
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