Re: [RFC][PATCH] TIMER_BH-less smptimers

[Anton Blanchard <anton@samba.org>]

Hi Dipankar,
 
> The tasklet code also needs fixing. It is a miracle that the kernel
> booted when I tested that code. Here is a fixed diff.

:) I was surprised it worked with the missing spin_unlock too. Im
testing the fixed diff now, so far it looks good.

> I am curious about performance of smptimers. It seems that
> webserver benchmark performance worsens with smptimers (Ingo version)
> contrary to our expectations. Do you see this ? If so, could this
> happen because -
> 
> 1) Bouncing around of global_bh_lock cacheline by more cpus compared
> to earlier timer implemenation ?
> 2) All per-cpu timers invoked from timer_bh running in one cpu ?
> 
> Do you see any other side-effects of smptimers ?

We used to see bad behaviour. It turned out to be the per cpu
timer interrupt firing at exactly the same time on all cpus. One
cpu would successfully spin_trylock and the others would fail
and postpone the work.

We now evenly space the per cpu interrupts. Does intel do the same?

> Also, did my PPC changes for smptimers work or you had to fix it ?

I tested ppc64 and it worked fine.

Anton