Re: Optimizing notifier_call_chain

From: Alan Stern
Date: Wed Nov 23 2005 - 11:01:00 EST

On Tue, 22 Nov 2005, Chandra Seetharaman wrote:

> Hello,
> While making the notifier chain safe, i found room for some optimization.
> Please comment on if it is worth pursuing.
> notifier_call_chain() calls registered callouts for _all_ events. But, many
> of the callouts handle only few events.
> If we change notifier_call_chain() to call the callout only on registered
> events, we can avoid few function overhead.
> Currently, events is of free format, we have to make it bit per event.
> Among the existing users, ia64die_chain uses the highest number(25) of
> events. i think 64 bit event would suffice.
> simplified logic:
> notifier_call_chain(event)
> {
> if ((head->event & event) != 0)
> return;
> for_each_callout {
> if ((notifier_block->event & event) != 0)
> notifier_block->call(event);
> }
> }

You would need to mask against (1ull << event), not against event.
tset_bit() might work better.

Do you really want to do this? It will mean changing every single
notifier_block definition in the kernel, in addition to all the the
notifier_heads. That's an awful lot of work for a relatively small gain.
I believe that notifier_call_chain does not run very often, but I don't
have any actual figures.

Alan Stern

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