Re: [PATCH 4/4] writeback, memcg: Implement foreign dirty flushing

Andrew Morton
2019-08-06T23:03:11Z

On Sat, 3 Aug 2019 07:01:55 -0700 Tejun Heo <tj@kernel.org> wrote:

> There's an inherent mismatch between memcg and writeback. The former
> trackes ownership per-page while the latter per-inode. This was a
> deliberate design decision because honoring per-page ownership in the
> writeback path is complicated, may lead to higher CPU and IO overheads
> and deemed unnecessary given that write-sharing an inode across
> different cgroups isn't a common use-case.
>
> Combined with inode majority-writer ownership switching, this works
> well enough in most cases but there are some pathological cases. For
> example, let's say there are two cgroups A and B which keep writing to
> different but confined parts of the same inode. B owns the inode and
> A's memory is limited far below B's. A's dirty ratio can rise enough
> to trigger balance_dirty_pages() sleeps but B's can be low enough to
> avoid triggering background writeback. A will be slowed down without
> a way to make writeback of the dirty pages happen.
>
> This patch implements foreign dirty recording and foreign mechanism so
> that when a memcg encounters a condition as above it can trigger
> flushes on bdi_writebacks which can clean its pages. Please see the
> comment on top of mem_cgroup_track_foreign_dirty_slowpath() for
> details.
>
> ...
>
> +void mem_cgroup_track_foreign_dirty_slowpath(struct page *page,
> + struct bdi_writeback *wb)
> +{
> + struct mem_cgroup *memcg = page->mem_cgroup;
> + struct memcg_cgwb_frn *frn;
> + u64 now = jiffies_64;
> + u64 oldest_at = now;
> + int oldest = -1;
> + int i;
> +
> + /*
> + * Pick the slot to use. If there is already a slot for @wb, keep
> + * using it. If not replace the oldest one which isn't being
> + * written out.
> + */
> + for (i = 0; i < MEMCG_CGWB_FRN_CNT; i++) {
> + frn = &memcg->cgwb_frn[i];
> + if (frn->bdi_id == wb->bdi->id &&
> + frn->memcg_id == wb->memcg_css->id)
> + break;
> + if (frn->at < oldest_at && atomic_read(&frn->done.cnt) == 1) {
> + oldest = i;
> + oldest_at = frn->at;
> + }
> + }
> +
> + if (i < MEMCG_CGWB_FRN_CNT) {
> + unsigned long update_intv =
> + min_t(unsigned long, HZ,
> + msecs_to_jiffies(dirty_expire_interval * 10) / 8);

An explanation of what's going on here would be helpful.

Why "* 1.25" and not, umm "* 1.24"?

> + /*
> + * Re-using an existing one. Let's update timestamp lazily
> + * to avoid making the cacheline hot.
> + */
> + if (frn->at < now - update_intv)
> + frn->at = now;
> + } else if (oldest >= 0) {
> + /* replace the oldest free one */
> + frn = &memcg->cgwb_frn[oldest];
> + frn->bdi_id = wb->bdi->id;
> + frn->memcg_id = wb->memcg_css->id;
> + frn->at = now;
> + }
> +}
> +
> +/*
> + * Issue foreign writeback flushes for recorded foreign dirtying events
> + * which haven't expired yet and aren't already being written out.
> + */
> +void mem_cgroup_flush_foreign(struct bdi_writeback *wb)
> +{
> + struct mem_cgroup *memcg = mem_cgroup_from_css(wb->memcg_css);
> + unsigned long intv = msecs_to_jiffies(dirty_expire_interval * 10);

Ditto.

> + u64 now = jiffies_64;
> + int i;
> +
> + for (i = 0; i < MEMCG_CGWB_FRN_CNT; i++) {
> + struct memcg_cgwb_frn *frn = &memcg->cgwb_frn[i];
> +
> + if (frn->at > now - intv && atomic_read(&frn->done.cnt) == 1) {
> + frn->at = 0;
> + cgroup_writeback_by_id(frn->bdi_id, frn->memcg_id,
> + LONG_MAX, WB_REASON_FOREIGN_FLUSH,
> + &frn->done);
> + }
> + }
> +}
> +