Re: [PATCH 6/8] kstaled: rate limit pages scanned per second.

Andrew Morton
2011-09-22T23:15:39Z

On Fri, 16 Sep 2011 20:39:11 -0700
Michel Lespinasse <walken@google.com> wrote:

> Scan some number of pages from each node every second, instead of trying to
> scan the entime memory at once and being idle for the rest of the configured
> interval.

Well... why? The amount of work done per scan interval is the same
(actually, it will be slightly increased due to cache evictions).

I think we should see a good explanation of what observed problem this
hackery^Wtweak is trying to solve. Once that is revealed, we can
compare the proposed solution with one based on thread policy/priority
(for example).

>
> ....
>
> @@ -5788,21 +5800,60 @@ static int kstaled(void *dummy)
> */
> BUG_ON(scan_seconds <= 0);
>
> - for_each_mem_cgroup_all(mem)
> - memset(&mem->idle_scan_stats, 0,
> - sizeof(mem->idle_scan_stats));
> + earlier = jiffies;
>
> + scan_done = true;
> for_each_node_state(nid, N_HIGH_MEMORY)
> - kstaled_scan_node(NODE_DATA(nid));
> + scan_done &= kstaled_scan_node(NODE_DATA(nid),
> + scan_seconds, reset);
> +
> + if (scan_done) {
> + struct mem_cgroup *mem;
> +
> + for_each_mem_cgroup_all(mem) {
> + write_seqcount_begin(&mem->idle_page_stats_lock);
> + mem->idle_page_stats = mem->idle_scan_stats;
> + mem->idle_page_scans++;
> + write_seqcount_end(&mem->idle_page_stats_lock);
> + memset(&mem->idle_scan_stats, 0,
> + sizeof(mem->idle_scan_stats));
> + }
> + }
>
> - for_each_mem_cgroup_all(mem) {
> - write_seqcount_begin(&mem->idle_page_stats_lock);
> - mem->idle_page_stats = mem->idle_scan_stats;
> - mem->idle_page_scans++;
> - write_seqcount_end(&mem->idle_page_stats_lock);
> + delta = jiffies - earlier;
> + if (delta < HZ / 2) {
> + delayed = 0;
> + schedule_timeout_interruptible(HZ - delta);
> + } else {
> + /*
> + * Emergency throttle if we're taking too long.
> + * We are supposed to scan an entire slice in 1 second.
> + * If we keep taking longer for 10 consecutive times,
> + * scale back our scan_seconds.
> + *
> + * If someone changed kstaled_scan_seconds while we
> + * were running, hope they know what they're doing and
> + * assume they've eliminated any delays.
> + */
> + bool updated = false;
> + spin_lock(&kstaled_scan_seconds_lock);
> + if (scan_seconds != kstaled_scan_seconds)
> + delayed = 0;
> + else if (++delayed == 10) {
> + delayed = 0;
> + scan_seconds *= 2;
> + kstaled_scan_seconds = scan_seconds;
> + updated = true;
> + }
> + spin_unlock(&kstaled_scan_seconds_lock);
> + if (updated)
> + pr_warning("kstaled taking too long, "
> + "scan_seconds now %d\n",
> + scan_seconds);
> + schedule_timeout_interruptible(HZ / 2);

This is all rather unpleasing.

>
> - schedule_timeout_interruptible(scan_seconds * HZ);
> + reset = scan_done;
> }