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

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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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