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SubjectRe: [PATCH -V3 01/11] percpu_counters: make fbc->count read atomic on 32 bit architecture
On Wed, Aug 27, 2008 at 12:05:53PM -0700, Andrew Morton wrote:
> On Wed, 27 Aug 2008 20:58:26 +0530
> "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com> wrote:
>
> > fbc->count is of type s64. The change was introduced by
> > 0216bfcffe424a5473daa4da47440881b36c1f4 which changed the type
> > from long to s64. Moving to s64 also means on 32 bit architectures
> > we can get wrong values on fbc->count. Since fbc->count is read
> > more frequently and updated rarely use seqlocks. This should
> > reduce the impact of locking in the read path for 32bit arch.
> >
>
> So... yesterday's suggestionm to investigate implementing this at a
> lower level wasn't popular?

I wanted to sent the entire patchset which fixes ENOSPC issues with
delalloc. It happened to be on the next day you looked at the previous
mail. Sending the patch again in now way mean we should not have
generic atomic64_t;


>
> > include/linux/percpu_counter.h | 28 ++++++++++++++++++++++++----
> > lib/percpu_counter.c | 20 ++++++++++----------
> > 2 files changed, 34 insertions(+), 14 deletions(-)
> >
> > diff --git a/include/linux/percpu_counter.h b/include/linux/percpu_counter.h
> > index 9007ccd..1b711a1 100644
> > --- a/include/linux/percpu_counter.h
> > +++ b/include/linux/percpu_counter.h
> > @@ -6,7 +6,7 @@
> > * WARNING: these things are HUGE. 4 kbytes per counter on 32-way P4.
> > */
> >
> > -#include <linux/spinlock.h>
> > +#include <linux/seqlock.h>
> > #include <linux/smp.h>
> > #include <linux/list.h>
> > #include <linux/threads.h>
> > @@ -16,7 +16,7 @@
> > #ifdef CONFIG_SMP
> >
> > struct percpu_counter {
> > - spinlock_t lock;
> > + seqlock_t lock;
> > s64 count;
> > #ifdef CONFIG_HOTPLUG_CPU
> > struct list_head list; /* All percpu_counters are on a list */
> > @@ -53,10 +53,30 @@ 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
> > +/* doesn't have atomic 64 bit operation */
> > +static inline s64 fbc_count(struct percpu_counter *fbc)
> > +{
> > + s64 ret;
> > + unsigned seq;
> > + do {
> > + seq = read_seqbegin(&fbc->lock);
> > + ret = fbc->count;
> > + } while (read_seqretry(&fbc->lock, seq));
> > + return ret;
> > +
>
> Please don't put unneeded blank lines into random places.
>

Will fix

> > +}
> > +#endif
>
> This is now too large to be inlined.
>


How do we actually figure that out ? I have been making that mistakes
quiet often.

> > +static inline s64 percpu_counter_read(struct percpu_counter *fbc)
> > +{
> > + return fbc_count(fbc);
> > +}
>

-aneesh


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