Re: [PATCH] cgroup: rstat: Simplified cgroup_base_stat_flush() update last_bstat logic

[Hao Jia <jiahao.os@bytedance.com>]

On 2023/5/18 Hao Jia wrote:
> In cgroup_base_stat_flush() function, {rstatc, cgrp}->last_bstat
> needs to be updated to the current {rstatc, cgrp}->bstat, directly
> assigning values instead of adding the last value to delta.
> 
> Signed-off-by: Hao Jia <jiahao.os@bytedance.com>
> ---
>   kernel/cgroup/rstat.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c
> index 9c4c55228567..3e5c4c1c92c6 100644
> --- a/kernel/cgroup/rstat.c
> +++ b/kernel/cgroup/rstat.c
> @@ -376,14 +376,14 @@ static void cgroup_base_stat_flush(struct cgroup *cgrp, int cpu)
>   	/* propagate percpu delta to global */
>   	cgroup_base_stat_sub(&delta, &rstatc->last_bstat);  *(1)*
>   	cgroup_base_stat_add(&cgrp->bstat, &delta);
> -	cgroup_base_stat_add(&rstatc->last_bstat, &delta);
> +	rstatc->last_bstat = rstatc->bstat;		    *(2)*

Some things are wrong, the value of rstatc->bstat at (1) and (2) may not 
be the same, rstatc->bstat may be updated on other cpu. Sorry for the noise.

>   
>   	/* propagate global delta to parent (unless that's root) */
>   	if (cgroup_parent(parent)) {
>   		delta = cgrp->bstat;
>   		cgroup_base_stat_sub(&delta, &cgrp->last_bstat);
>   		cgroup_base_stat_add(&parent->bstat, &delta);
> -		cgroup_base_stat_add(&cgrp->last_bstat, &delta);
> +		cgrp->last_bstat = cgrp->bstat;
>   	}
>   }
>   

Maybe something like this?


In cgroup_base_stat_flush() function, {rstatc, cgrp}->last_bstat
needs to be updated to the current {rstatc, cgrp}->bstat after the
calculation.

For the rstatc->last_bstat case, rstatc->bstat may be updated on other
cpus during our calculation, resulting in inconsistent rstatc->bstat
statistics for the two reads. So we use the temporary variable @cur to
record the read statc->bstat statistics, and use @cur to update
rstatc->last_bstat.

For the cgrp->last_bstat case, we already hold cgroup_rstat_lock, so
cgrp->bstat will not change during the calculation process, and it can
be directly used to update cgrp->last_bstat.

It is better for us to assign directly instead of using
cgroup_base_stat_add() to update {rstatc, cgrp}->last_bstat.

Signed-off-by: Hao Jia <jiahao.os@bytedance.com>
---
  kernel/cgroup/rstat.c | 9 +++++----
  1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c
index 9c4c55228567..17a6a1fcc2d4 100644
--- a/kernel/cgroup/rstat.c
+++ b/kernel/cgroup/rstat.c
@@ -360,7 +360,7 @@ static void cgroup_base_stat_flush(struct cgroup 
*cgrp, int cpu)
  {
  	struct cgroup_rstat_cpu *rstatc = cgroup_rstat_cpu(cgrp, cpu);
  	struct cgroup *parent = cgroup_parent(cgrp);
-	struct cgroup_base_stat delta;
+	struct cgroup_base_stat delta, cur;
  	unsigned seq;

  	/* Root-level stats are sourced from system-wide CPU stats */
@@ -370,20 +370,21 @@ static void cgroup_base_stat_flush(struct cgroup 
*cgrp, int cpu)
  	/* fetch the current per-cpu values */
  	do {
  		seq = __u64_stats_fetch_begin(&rstatc->bsync);
-		delta = rstatc->bstat;
+		cur = rstatc->bstat;
  	} while (__u64_stats_fetch_retry(&rstatc->bsync, seq));

  	/* propagate percpu delta to global */
+	delta = cur;
  	cgroup_base_stat_sub(&delta, &rstatc->last_bstat);
  	cgroup_base_stat_add(&cgrp->bstat, &delta);
-	cgroup_base_stat_add(&rstatc->last_bstat, &delta);
+	rstatc->last_bstat = cur;

  	/* propagate global delta to parent (unless that's root) */
  	if (cgroup_parent(parent)) {
  		delta = cgrp->bstat;
  		cgroup_base_stat_sub(&delta, &cgrp->last_bstat);
  		cgroup_base_stat_add(&parent->bstat, &delta);
-		cgroup_base_stat_add(&cgrp->last_bstat, &delta);
+		cgrp->last_bstat = cgrp->bstat;
  	}
  }