Messages in this thread |  | | Date | Thu, 16 Apr 2015 20:02:27 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH 1/3] sched, timer: Remove usages of ACCESS_ONCE in the scheduler |
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* Peter Zijlstra <peterz@infradead.org> wrote:
> On Wed, Apr 15, 2015 at 09:46:01AM +0200, Ingo Molnar wrote: > > > @@ -2088,7 +2088,7 @@ void task_numa_fault(int last_cpupid, int mem_node, int pages, int flags) > > > > static void reset_ptenuma_scan(struct task_struct *p) > > { > > - ACCESS_ONCE(p->mm->numa_scan_seq)++; > > + WRITE_ONCE(p->mm->numa_scan_seq, READ_ONCE(p->mm->numa_scan_seq) + 1); > > vs > > seq = ACCESS_ONCE(p->mm->numa_scan_seq); > if (p->numa_scan_seq == seq) > return; > p->numa_scan_seq = seq; > > > > So the original ACCESS_ONCE() barriers were misguided to begin with: I > > think they tried to handle races with the scheduler balancing softirq > > and tried to avoid having to use atomics for the sequence counter > > (which would be overkill), but things like ACCESS_ONCE(x)++ never > > guaranteed atomicity (or even coherency) of the update. > > > > But since in reality this is only statistical sampling code, all these > > compiler barriers can be removed I think. Peter, Mel, Rik, do you > > agree? > > ACCESS_ONCE() is not a compiler barrier
It's not a general compiler barrier (and I didn't claim so) but it is still a compiler barrier: it's documented as a weak, variable specific barrier in Documentation/memor-barriers.txt:
COMPILER BARRIER ----------------
The Linux kernel has an explicit compiler barrier function that prevents the compiler from moving the memory accesses either side of it to the other side:
barrier();
This is a general barrier -- there are no read-read or write-write variants of barrier(). However, ACCESS_ONCE() can be thought of as a weak form for barrier() that affects only the specific accesses flagged by the ACCESS_ONCE().
[...]
> The 'read' side uses ACCESS_ONCE() for two purposes: > - to load the value once, we don't want the seq number to change under > us for obvious reasons > - to avoid load tearing and observe weird seq numbers > > The update side uses ACCESS_ONCE() to avoid write tearing, and > strictly speaking it should also worry about read-tearing since its > not hard serialized, although its very unlikely to actually have > concurrency (IIRC).
So what bad effects can there be from the very unlikely read and write tearing?
AFAICS nothing particularly bad. On the read side:
seq = ACCESS_ONCE(p->mm->numa_scan_seq); if (p->numa_scan_seq == seq) return; p->numa_scan_seq = seq;
If p->mm->numa_scan_seq gets loaded twice (very unlikely), and two different values happen, then we might get a 'double' NUMA placement run - i.e. statistical noise.
On the update side:
ACCESS_ONCE(p->mm->numa_scan_seq)++; p->mm->numa_scan_offset = 0;
If the compiler tears that up we might skip an update - again statistical noise at worst.
Nor is compiler tearing the only theoretical worry here: in theory, with long cache coherency latencies we might get two updates 'mixed up' and resulting in a (single) missed update.
Only atomics would solve all the races, but I think that would be overdoing it.
This is what I meant by that there's no harm from this race.
Thanks,
Ingo
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