> > I'm worried about two things:
> >
> > 1) If the per-bdi threshold becomes smaller than the granularity of
> > the per-bdi stat (due to the per-CPU counters), then things will
> > break. Shouldn't there be some sanity checking for the calculated
> > threshold?
>
> I'm not sure what you're referring to.
>
> void get_writeout_scale(struct backing_dev_info *bdi, int *scale, int *div)
> {
> int bits = vm_cycle_shift - 1;
> unsigned long total = __global_bdi_stat(BDI_WRITEOUT_TOTAL);
> unsigned long cycle = 1UL << bits;
> unsigned long mask = cycle - 1;
>
> if (bdi_cap_writeback_dirty(bdi)) {
> bdi_writeout_norm(bdi);
> *scale = __bdi_stat(bdi, BDI_WRITEOUT);
> } else
> *scale = 0;
>
> *div = cycle + (total & mask);
> }
>
> where cycle ~ vm_total_pages
> scale can be a tad off due to overstep here:
>
> void __inc_bdi_stat(struct backing_dev_info *bdi, enum bdi_stat_item item)
> {
> struct bdi_per_cpu_data *pcd = &bdi->pcd[smp_processor_id()];
> s8 *p = pcd->bdi_stat_diff + item;
>
> (*p)++;
>
> if (unlikely(*p > pcd->stat_threshold)) {
> int overstep = pcd->stat_threshold / 2;
>
> bdi_stat_add(*p + overstep, bdi, item);
> *p = -overstep;
> }
> }
>
> so it could be that: scale / cycle > 1
> by a very small amount; however:
No, I'm worried about the case when scale is too small. If the
per-bdi threshold becomes smaller than stat_threshold, then things
won't work, because dirty+writeback will never go below the threshold,
possibly resulting in the deadlock we are trying to avoid.
BTW, the second argument of get_dirty_limits() doesn't seem to get
used by the caller, or does it?
> here we clip to 'reserve' which is the total amount of dirty threshold
> not dirty by others.
>
> > 2) The loop is sleeping in congestion_wait(WRITE), which seems wrong.
> > It may well be possible that none of the queues are congested, so
> > it will sleep the full .1 second. But by that time the queue may
> > have become idle and is just sitting there doing nothing. Maybe
> > there should be a per-bdi waitq, that is woken up, when the per-bdi
> > stats are updated.
>
> Good point, .1 seconds is a lot of time.
>
> I'll cook up something like that if nobody beats me to it :-)
I realized, that it's maybe worth storing last the threshold in the
bdi as well, so that balance_dirty_pages() doesn't get woken up too
many times unnecessarilty. But I don't know...
Thanks,
Miklos
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