linux-kernel - Re: [PATCH v2] sched/rt: fix the case where sched_rt_period

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Date:   Tue, 17 May 2022 01:55:41 +0000
From:   "Yajun Deng" <yajun.deng@...ux.dev>
To:     "Valentin Schneider" <vschneid@...hat.com>, mingo@...hat.com,
        peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] sched/rt: fix the case where sched_rt_period_us is
 negative

May 16, 2022 11:04 PM, "Valentin Schneider" <vschneid@...hat.com> wrote:

> On 12/05/22 08:39, Yajun Deng wrote:
> 
>> The proc_dointvec() is for integer, but sysctl_sched_rt_period is a
>> unsigned integer, proc_dointvec() would convert negative number into
>> positive number. So both proc_dointvec() and sched_rt_global_validate()
>> aren't return error even if we set a negative number.
>> 
>> Use proc_dointvec_minmax() instead of proc_dointvec() and use extra1
>> limit the minimum value for sched_rt_period_us/sched_rt_runtime_us.
>> 
>> Fixes: 391e43da797a ("sched: Move all scheduler bits into kernel/sched/")
> 
> That's just the last apparent change of the incriminated variable. AFAICT
> the issue stems from:
> 
> - sysctl_sched_rt_period being unsigned int
> - the ctl entry using proc_dointvec()
> - the bounds check on sysctl_sched_rt_period being just <= 0 which doesn't
> actually respect the [1, INT_MAX] stated in
> Documentation/scheduler/sched-rt-group.rst
> 
> The <= thing was added by:
> 
> ec5d498991e8 ("sched: fix deadlock in setting scheduler parameter to zero")
> 
> but AFAICT the unsigned int vs proc_dointvec() thing dates back to:
> 
> d0b27fa77854 ("sched: rt-group: synchonised bandwidth period")
> 

I know that, but I didn't find out the source. Thank you for helping me find out it.

>> Signed-off-by: Yajun Deng <yajun.deng@...ux.dev>
>> ---
> 
> In the absence of a cover letter (e.g. single-patch submission), this is
> where you should write patch version changelogs (see
> Documentation/process).
> 

Got it, I will add it to the next version.

>> kernel/sched/rt.c | 8 ++++----
>> 1 file changed, 4 insertions(+), 4 deletions(-)
>> 
>> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
>> index b491a0f8c25d..3add32679885 100644
>> --- a/kernel/sched/rt.c
>> +++ b/kernel/sched/rt.c
>> @@ -37,6 +37,7 @@ static struct ctl_table sched_rt_sysctls[] = {
>> .maxlen = sizeof(unsigned int),
>> .mode = 0644,
>> .proc_handler = sched_rt_handler,
>> + .extra1 = SYSCTL_ONE,
> 
> Per earlier comment, the Documentation says that this needs to be within
> [1, INT_MAX], which you do get by excluding negative values when casting to
> int...
> 
> How about we make sysctl_sched_rt_period int on top of this, then all variables
> modified by the sched_rt_handler() proc_dointvec() are *actually* int, and
> the upper bound requires less mental gymnastics to be figured out?
> 

Yes, we can make sysctl_sched_rt_period int.
>> },
>> {
>> .procname = "sched_rt_runtime_us",
>> @@ -44,6 +45,8 @@ static struct ctl_table sched_rt_sysctls[] = {
>> .maxlen = sizeof(int),
>> .mode = 0644,
>> .proc_handler = sched_rt_handler,
>> + .extra1 = SYSCTL_NEG_ONE,
>> + .extra2 = (void *)&sysctl_sched_rt_period,
> 
> Per this, you could also remove the
> 
> ((sysctl_sched_rt_runtime > sysctl_sched_rt_period) ||
> 
> from sched_rt_global_validate(), no?
>

No, the extra2 just limit the maximum value of sysctl_sched_rt_runtime is sysctl_sched_rt_period, but not limit the minimum value of sysctl_sched_rt_period is sysctl_sched_rt_runtime. (sysctl_sched_rt_runtime > sysctl_sched_rt_period) can do both. 
Its purpose is to return error earlier. Perhaps I should remove extra2 to avoid ambiguity.

>> }, 
>> {
>> .procname = "sched_rr_timeslice_ms",
>> @@ -2959,9 +2962,6 @@ static int sched_rt_global_constraints(void)
>> #ifdef CONFIG_SYSCTL
>> static int sched_rt_global_validate(void)
>> {
>> - if (sysctl_sched_rt_period <= 0)
>> - return -EINVAL;
>> -
>> if ((sysctl_sched_rt_runtime != RUNTIME_INF) &&
>> ((sysctl_sched_rt_runtime > sysctl_sched_rt_period) ||
>> ((u64)sysctl_sched_rt_runtime *
>> @@ -2992,7 +2992,7 @@ static int sched_rt_handler(struct ctl_table *table, int write, void *buffer,
>> old_period = sysctl_sched_rt_period;
>> old_runtime = sysctl_sched_rt_runtime;
>> 
>> - ret = proc_dointvec(table, write, buffer, lenp, ppos);
>> + ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
>> 
>> if (!ret && write) {
>> ret = sched_rt_global_validate();
>> --
>> 2.25.1