Messages in this thread Patch in this message | | | Date | Mon, 15 Jul 2019 14:38:01 +0100 | From | Patrick Bellasi <> | Subject | Re: [PATCH v11 1/5] sched/core: uclamp: Extend CPU's cgroup controller |
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On 08-Jul 12:08, Quentin Perret wrote: > Hi Patrick,
Hi Quentin!
> On Monday 08 Jul 2019 at 09:43:53 (+0100), Patrick Bellasi wrote: > > +static inline int uclamp_scale_from_percent(char *buf, u64 *value) > > +{ > > + *value = SCHED_CAPACITY_SCALE; > > + > > + buf = strim(buf); > > + if (strncmp("max", buf, 4)) { > > + s64 percent; > > + int ret; > > + > > + ret = cgroup_parse_float(buf, 2, &percent); > > + if (ret) > > + return ret; > > + > > + percent <<= SCHED_CAPACITY_SHIFT; > > + *value = DIV_ROUND_CLOSEST_ULL(percent, 10000); > > + } > > + > > + return 0; > > +} > > + > > +static inline u64 uclamp_percent_from_scale(u64 value) > > +{ > > + return DIV_ROUND_CLOSEST_ULL(value * 10000, SCHED_CAPACITY_SCALE); > > +} > > FWIW, I tried the patches and realized these conversions result in a > 'funny' behaviour from a user's perspective. Things like this happen: > > $ echo 20 > cpu.uclamp.min > $ cat cpu.uclamp.min > 20.2 > $ echo 20.2 > cpu.uclamp.min > $ cat cpu.uclamp.min > 20.21 > > Having looked at the code, I get why this is happening, but I'm not sure > if a random user will. It's not an issue per se, but it's just a bit > weird.
Yes, that's what we get if we need to use a "two decimal digit precision percentage" to represent a 1024 range in kernel space.
I don't think the "percent <=> utilization" conversion code can be made more robust. The only possible alternative I see to get back exactly what we write in, is to store the actual request in kernel space, alongside its conversion to the SCHED_CAPACITY_SCALE required by the actual scheduler code.
Something along these lines (on top of what we have in this series):
---8<--- diff --git a/kernel/sched/core.c b/kernel/sched/core.c index ddc5fcd4b9cf..82b28cfa5c3f 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -7148,40 +7148,35 @@ static void cpu_util_update_eff(struct cgroup_subsys_state *css) } }
-static inline int uclamp_scale_from_percent(char *buf, u64 *value) +static inline int uclamp_scale_from_percent(char *buf, s64 *percent, u64 *scale) { - *value = SCHED_CAPACITY_SCALE; + *scale = SCHED_CAPACITY_SCALE;
buf = strim(buf); if (strncmp("max", buf, 4)) { - s64 percent; int ret;
- ret = cgroup_parse_float(buf, 2, &percent); + ret = cgroup_parse_float(buf, 2, percent); if (ret) return ret;
- percent <<= SCHED_CAPACITY_SHIFT; - *value = DIV_ROUND_CLOSEST_ULL(percent, 10000); + *scale = *percent << SCHED_CAPACITY_SHIFT; + *scale = DIV_ROUND_CLOSEST_ULL(*scale, 10000); }
return 0; }
-static inline u64 uclamp_percent_from_scale(u64 value) -{ - return DIV_ROUND_CLOSEST_ULL(value * 10000, SCHED_CAPACITY_SCALE); -} - static ssize_t cpu_uclamp_min_write(struct kernfs_open_file *of, char *buf, size_t nbytes, loff_t off) { struct task_group *tg; u64 min_value; + s64 percent; int ret;
- ret = uclamp_scale_from_percent(buf, &min_value); + ret = uclamp_scale_from_percent(buf, &percent, &min_value); if (ret) return ret; if (min_value > SCHED_CAPACITY_SCALE) @@ -7197,6 +7192,9 @@ static ssize_t cpu_uclamp_min_write(struct kernfs_open_file *of, /* Update effective clamps to track the most restrictive value */ cpu_util_update_eff(of_css(of));
+ /* Keep track of the actual requested value */ + tg->uclamp_pct[UCLAMP_MIN] = percent; + rcu_read_unlock(); mutex_unlock(&uclamp_mutex);
@@ -7209,9 +7207,10 @@ static ssize_t cpu_uclamp_max_write(struct kernfs_open_file *of, { struct task_group *tg; u64 max_value; + s64 percent; int ret;
- ret = uclamp_scale_from_percent(buf, &max_value); + ret = uclamp_scale_from_percent(buf, &percent, &max_value); if (ret) return ret; if (max_value > SCHED_CAPACITY_SCALE) @@ -7227,6 +7226,9 @@ static ssize_t cpu_uclamp_max_write(struct kernfs_open_file *of, /* Update effective clamps to track the most restrictive value */ cpu_util_update_eff(of_css(of));
+ /* Keep track of the actual requested value */ + tg->uclamp_pct[UCLAMP_MAX] = percent; + rcu_read_unlock(); mutex_unlock(&uclamp_mutex);
@@ -7251,7 +7253,7 @@ static inline void cpu_uclamp_print(struct seq_file *sf, return; }
- percent = uclamp_percent_from_scale(util_clamp); + percent = tg->uclamp_pct[clamp_id]; percent = div_u64_rem(percent, 100, &rem); seq_printf(sf, "%llu.%u\n", percent, rem); } diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index 0e37f4a4e536..4f9b0c660310 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -395,6 +395,8 @@ struct task_group { struct cfs_bandwidth cfs_bandwidth;
#ifdef CONFIG_UCLAMP_TASK_GROUP + /* The two decimal precision [%] value requested from user-space */ + unsigned int uclamp_pct[UCLAMP_CNT]; /* Clamp values requested for a task group */ struct uclamp_se uclamp_req[UCLAMP_CNT]; /* Effective clamp values used for a task group */ ---8<--- > I guess one way to fix this would be to revert back to having a > 1024-scale for the cgroup interface too ... Though I understand Tejun > wanted % for consistency with other things.
Yes that would be another option, which will also keep aligned the per-task and system-wide APIs with the CGroups one. Although, AFAIU, having two different APIs is not considered a major issue. > So, I'm not sure if this is still up for discussion, but in any case I > wanted to say I support your original idea of using a 1024-scale for the > cgroups interface, since that would solve the 'issue' above and keeps > things consistent with the per-task API too.
Right, I'm personally more leaning toward either going back to use SCHED_CAPACITY_SCALE or the add the small change I suggested above.
Tejun, Peter: any preference? Alternative suggestions?
> Thanks, > Quentin
Cheers, Patrick
-- #include <best/regards.h>
Patrick Bellasi
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