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

[Jan Kara]

On Sat 03-08-19 07:01:55, Tejun Heo wrote:
> +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);
> +		/*
> +		 * 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;
> +	}

I have to say I'm a bit nervous about the completely lockless handling
here. I understand that garbage in the cgwb_frn will just result in this
mechanism not working and possibly flushing wrong wb's but still it seems a
bit fragile. But I don't see any cheap way of synchronizing this so I guess
let's try how this will work in practice.

								Honza
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR