On Fri, Dec 02, 2011 at 07:06:22PM +0900, KAMEZAWA Hiroyuki wrote:
> I'm now testing this patch, removing PCG_ACCT_LRU, onto mmotm.
> How do you think ?
> @@ -1024,18 +1026,8 @@ void mem_cgroup_lru_del_list(struct page *page, enum lru_list lru)
> return;
>
> pc = lookup_page_cgroup(page);
> - /*
> - * root_mem_cgroup babysits uncharged LRU pages, but
> - * PageCgroupUsed is cleared when the page is about to get
> - * freed. PageCgroupAcctLRU remembers whether the
> - * LRU-accounting happened against pc->mem_cgroup or
> - * root_mem_cgroup.
> - */
> - if (TestClearPageCgroupAcctLRU(pc)) {
> - VM_BUG_ON(!pc->mem_cgroup);
> - memcg = pc->mem_cgroup;
> - } else
> - memcg = root_mem_cgroup;
> + memcg = pc->mem_cgroup ? pc->mem_cgroup : root_mem_cgroup;
> + VM_BUG_ON(memcg != pc->mem_cgroup_lru);
> mz = page_cgroup_zoneinfo(memcg, page);
> /* huge page split is done under lru_lock. so, we have no races. */
> MEM_CGROUP_ZSTAT(mz, lru) -= 1 << compound_order(page);
Nobody clears pc->mem_cgroup upon uncharge, so this may end up
mistakenly lru-unaccount a page that was never charged against the
stale pc->mem_cgroup (e.g. a swap readahead page that has not been
charged yet gets isolated by reclaim).
On the other hand, pages that were uncharged just before the lru_del
MUST be lru-unaccounted against pc->mem_cgroup.
PageCgroupAcctLRU made it possible to tell those two scenarios apart.
A possible solution could be to clear pc->mem_cgroup when the page is
finally freed so that only pages that have been charged since their
last allocation have pc->mem_cgroup set. But this means that the page
freeing hotpath will have to grow a lookup_page_cgroup(), amortizing
the winnings at least to some extent.
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@xxxxxxxxx. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/
Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>