Hi, 在 2024/06/28 4:10, Tejun Heo 写道:
Hello, On Thu, Jun 27, 2024 at 05:08:56PM +0800, Li Lingfeng wrote:The list corruption described in commit 6da668063279 ("blk-cgroup: fix list corruption from resetting io stat") has no effect. It's unnecessary to fix it.I find this paragraph a bit confusing. At the time, it was broken, right? And if we were to memset() now, it'd break again.As for cgroup v1, it does not use iostat any more after commit ad7c3b41e86b("blk-throttle: Fix io statistics for cgroup v1"), so using memset to clear iostat has no real effect.Ah, okay, this is because we made the stats blk-throtl specific but didn't implement ->pd_reset_stat_fn(), right?
I'm afraid not... Implement pd_reset_stat_fn() or not is another problem, this patch should be just code cleanup, not fixing any real problems.
As for cgroup v2, it will not call blkcg_reset_stats() to corrupt the list. The list of root cgroup can be used by both cgroup v1 and v2 while non-root cgroup can't since it must be removed before switch between cgroup v1 and v2. So it may has effect if the list of root used by cgroup v2 was corrupted after switching to cgroup v1, and switch back to cgroup v2 to use the corrupted list again. However, the root cgroup will not use the list any more after commit ef45fe470e1e("blk-cgroup: show global disk stats in root cgroup io.stat").Hmm... I'm still having a bit of trouble following this line of argument given that all the patch does is dropping stat clearing.@@ -668,7 +645,6 @@ static int blkcg_reset_stats(struct cgroup_subsys_state *css, * anyway. If you get hit by a race, retry. */ hlist_for_each_entry(blkg, &blkcg->blkg_list, blkcg_node) { - blkg_clear_stat(blkg); for (i = 0; i < BLKCG_MAX_POLS; i++) { struct blkcg_policy *pol = blkcg_policy[i];The patch looks fine to me although it'd be nice to follow up with a patch to implement ->pd_reset_stat_fn() for blk-throtl. I'm not quite following the list corruption part of argument.
The code deleted by this patch was claimed to fix a lsit corruption, however, the list corruption does not exist now hence related code can be removed: 1) Take a look at blk_cgroup_bio_start, now there are two conditions before this blkg can be added to the per_cpu list: blk_cgroup_bio_start if (!cgroup_subsys_on_dfl(io_cgrp_subsys)) -> the list will always be empty for cgroup v1 return; if (!cgroup_parent(blkcg->css.cgroup)) -> the list will always be empty for root blkg return; ... llist_add() 2) blkcg_reset_stats can only be called from cgroup v1 api, hence there is nothing to be cleared for blkg_clear_stat(); 3) Noted that user can switch from cgroup v2 to v1, however, we found that user must delete all the child cg to do that, hence only root blkg can be kept after switching to v1. And root blkg is bypassed from blk_cgroup_bio_start(), hence no problem. Thanks, Kuai
Thanks.
