On 22:00 Tue 16 Apr , David Rientjes wrote:
> On Tue, 16 Apr 2013, Oskar Andero wrote:
>
> > > > The comment in shrinker.h is misleading, not the source code.
> > > > do_shrinker_shrink() will fail for anything negative and 0.
> > >
> > > The comment is correct. The only acceptable negative return is -1.
> > > Look at the second time do_shrinker_shrink() is called from
> > > shrink_slab().
> > >
> > > 283 while (total_scan >= batch_size) {
> > > 284 int nr_before;
> > > 285
> > > 286 nr_before = do_shrinker_shrink(shrinker, shrink, 0);
> > > 287 shrink_ret = do_shrinker_shrink(shrinker, shrink,
> > > 288 batch_size);
> > > 289 if (shrink_ret == -1)
> > > 290 break;
> > > 291 if (shrink_ret < nr_before)
> > > 292 ret += nr_before - shrink_ret;
> > > 293 count_vm_events(SLABS_SCANNED, batch_size);
> >
> > Yes, the comment is correct with what is implemented in the code, but
> > that doesn't mean the code is right. IMHO, relaying on magical numbers is highly
> > questionable coding style.
> >
>
> No, it's not. This is controlled higher in shrink_slab() by this:
>
> max_pass = do_shrinker_shrink(shrinker, shrink, 0);
> if (max_pass <= 0)
> continue;
>
Sure, that looks ok, but that doesn't change the fact that line 289
above has a magical number and I guess that explains the comment:
> > > 289 if (shrink_ret == -1)
> > > 290 break;
Just to be clear - this is not about lowmemkiller, but rather a generic
clean-up of shrinkers that is needed IMO.
> and your patch is implemented incorrectly, i.e. it does not return
> LMK_BUSY if the spinlock is contended which needlessly recalls the
> shrinker later.
>
> You have a couple of options:
>
> - return -1 when the spinlock is contended immediately when
> !sc->nr_to_scan (although it should really be a cmpxchg since a
> spinlock isn't needed), or
I leave it to Snild to comment on the patch, but could you elaborate on why
you think cmpxchg is a better alternative than a spin_trylock? I just had a
brief look at the implementation for ARM and it looks like cmpxchg means
two unconditional memory barriers, whereas spin_trylock has one
conditional memory barrier. See arch/arm/include/asm/spinlock.h:
if (tmp == 0) {
smp_mb();
return 1;
} else {
return 0;
}
...and arch/arm/include/asm/cmpxchg.h:
smp_mb();
ret = __cmpxchg(ptr, old, new, size);
smp_mb();
AFAIK a memory barrier is pretty costly.
-Oskar
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