| title: | Re RFC PATCH v2 percpu counters make fbc |
|
On Mon, 2008-08-25 at 19:35 +0530, Aneesh Kumar K.V wrote:
On Mon, Aug 25, 2008 at 01:27:19PM +0200, Peter Zijlstra wrote:
On Mon, 2008-08-25 at 16:50 +0530, Aneesh Kumar K.V wrote:
@@ -53,10 +53,31 @@ static inline s64 percpu_counter_sum(struct percpu_counter *fbc)
return __percpu_counter_sum(fbc);
}
-static inline s64 percpu_counter_read(struct percpu_counter *fbc)
+#if BITS_PER_LONG == 64
+static inline s64 fbc_count(struct percpu_counter *fbc)
{
return fbc- count;
}
+#else
+/* doesnt have atomic 64 bit operation */
+static inline s64 fbc_count(struct percpu_counter *fbc)
+{
+ s64 ret;
+ unsigned seq;
+ unsigned long flags;
+ do {
+ seq = read_seqbegin_irqsave(&fbc- lock, flags);
+ ret = fbc- count;
+ } while(read_seqretry_irqrestore(&fbc- lock, seq, flags));
Do we really need to disabled IRQs here? It seems to me the worst that
can happen is that the IRQ will change - count and increase the sequence
number a bit - a case that is perfectly handled by the current retry
logic.
And not doing the IRQ flags bit saves a lot of time on some archs.
Will update in the next version. BTW does it make sense to do
the above unconditionally now ? ie to remove the #if ?. How much
impact would it be to do read_seqbegin and read_seqretry on a 64bit
machine too ?
theres a few smp_rmb()s in there - so that will at the very least be a
compiler barrier and thus generate slightly worse code along with the
few extra reads.
But Im not sure thats measurable..
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