Messages in this thread | | | Date | Mon, 1 Feb 2010 11:48:57 -0500 | From | Mathieu Desnoyers <> | Subject | Re: [patch 2/3] scheduler: add full memory barriers upon task switch at runqueue lock/unlock |
| |
* Linus Torvalds (torvalds@linux-foundation.org) wrote: > > > On Mon, 1 Feb 2010, Mathieu Desnoyers wrote: > > > > However, this does not deal with mm_cpumask update, and we cannot use > > the per-cpu rq lock, as it's a process-wide data structure updated with > > clear_bit/set_bit in switch_mm(). So at the very least, we would have to > > add memory barriers in switch_mm() on some architectures to deal with > > this. > > I'd much rather have a "switch_mm()" is a guaranteed memory barrier logic, > because quite frankly, I don't see how it ever couldn't be one anyway. It > fundamentally needs to do at least a TLB context switch (which may be just > switching an ASI around, not flushing the whole TLB, of course), and I bet > that for 99% of all architectures, that is already pretty much guaranteed > to be equivalent to a memory barrier. > > It certainly is for x86. "mov to cr0" is serializing (setting any control > register except cr8 is serializing). And I strongly suspect other > architectures will be too.
What we have to be careful about here is that it's not enough to just rely on switch_mm() containing a memory barrier. What we really need to enforce is that switch_mm() issues memory barriers both _before_ and _after_ mm_cpumask modification. The "after" part is usually dealt with by the TLB context switch, but the "before" part usually isn't.
> > Btw, one reason to strongly prefer "switch_mm()" over any random context > switch is that at least it won't affect inter-thread (kernel or user-land) > switching, including switching to/from the idle thread. > > So I'd be _much_ more open to a "let's guarantee that 'switch_mm()' always > implies a memory barrier" model than to playing clever games with > spinlocks.
If we really want to make this patch less intrusive, we can consider iterating on each online cpu in sys_membarrier() rather than on the mm_cpumask. But it comes at the cost of useless cache-line bouncing on large machines with few threads running in the process, as we would grab the rq locks one by one for all cpus.
Thanks,
Mathieu
-- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
| |