Re: VFS: file-max limit 50044 reached

[Linus Torvalds]

On Mon, 17 Oct 2005, Eric Dumazet wrote:
> 
> <lazy_mode=ON>
> Do we really need a TIF_RCUUPDATE flag, or could we just ask for a resched ?
> </lazy_mode>

Hmm.. Your patch looks very much like one I tried already, but the big 
difference being that I just cleared the count when doing the rcu 
callback. That was because I hadn't realized the importance of the 
maxbatch thing (so it didn't work for me, like it did for you).

Still - the actual RCU callback will only be called at the next timer tick 
or whatever as far as I can tell, so the first time you'll still have a 
_long_ RCU queue (and thus bad latency).

I guess that's inevitable - and TIF_RCUUPDATE wouldn't even help, because 
we still need to wait for the _other_ CPU's to get to their RCU quiescent 
event.

However, that leaves us with the nasty situation that we'll ve very 
inefficient: we'll do "maxbatch" RCU entries, and then return, and then 
force a whole re-schedule. That just can't be good.

How about instead of depending on "maxbatch", we'd depend on 
"need_resched()"? Mabe the "maxbatch" be a _minbatch_ thing, and then once 
we've done the minimum amount we _need_ to do (or emptied the RCU queue) 
we start honoring need_resched(), and return early if we do? 

That, together with your patch, should work, without causing ludicrous 
"reschedule every ten system calls" behaviour..

Hmm?

		Linus
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/