cpufreq, store_scaling_governor requires policy->rwsem to be held for duration of changing governors [v2]

On 08/04/2014 09:38 AM, Viresh Kumar wrote:
> On 4 August 2014 17:55, Prarit Bhargava <prarit@redhat.com> wrote:
>> The issue is the collision between the setup & teardown of the policy's governor
>> sysfs files.
>>
>> On creation the kernel does:
>>
>> down_write(&policy->rwsem)
>> mutex_lock(kernfs_mutex) <- note this is similar to the "old" sysfs_mutex.
>>
>> The opposite happens on a governor switch, specifically the existing governor's
>> exit, and then we get a lockdep warning.
> 
> Okay, probably a bit more clarity is what I was looking for. Suppose we try
> to change governor, now tell me what will happen.
> 
>> I tried to reproduce with the instructions but was unable to ... ut that was on
>> Friday ;) and I'm going to try again this morning.  I've also ping'd some of the
>> engineers here in the office who are working on ARM to get access to a system to
>> do further analysis and testing.
> 
> You DON'T need an ARM for that, just try that on any x86 machine which has
> multiple groups of CPUs sharing clock line. Or in other terms there are multiple
> policy structures there..

I do ... I really think I do.  Because this is all working on x86 AFAICT.

> 
> You just need to enable the flag we were discussing about, it just decided the
> location where governor's directory will get created. Nothing else.
> 

That doesn't appear to be correct.  I'm testing with the patch that removes the
locking workaround and:

as well as few printk statement sprinkled in the code.  I'm doing the following
and on *15* different x86 systems I do not see a problem:

My cpufreq related config is

#
# CPU Frequency scaling
#
CONFIG_CPU_FREQ=y
CONFIG_CPU_FREQ_GOV_COMMON=y
CONFIG_CPU_FREQ_STAT=m
CONFIG_CPU_FREQ_STAT_DETAILS=y
# CONFIG_CPU_FREQ_DEFAULT_GOV_PERFORMANCE is not set
# CONFIG_CPU_FREQ_DEFAULT_GOV_USERSPACE is not set
# CONFIG_CPU_FREQ_DEFAULT_GOV_ONDEMAND is not set
CONFIG_CPU_FREQ_DEFAULT_GOV_CONSERVATIVE=y
CONFIG_CPU_FREQ_GOV_PERFORMANCE=y
CONFIG_CPU_FREQ_GOV_POWERSAVE=y
CONFIG_CPU_FREQ_GOV_USERSPACE=y
CONFIG_CPU_FREQ_GOV_ONDEMAND=y
CONFIG_CPU_FREQ_GOV_CONSERVATIVE=y

I am doing (from boot)

[root@intel-canoepass-05 cpufreq]# cd /sys/devices/system/cpu/cpu2/cpufreq/
[root@intel-canoepass-05 cpufreq]# ls
affected_cpus     cpuinfo_transition_latency     scaling_driver
bios_limit        freqdomain_cpus                scaling_governor
conservative      related_cpus                   scaling_max_freq
cpuinfo_cur_freq  scaling_available_frequencies  scaling_min_freq
cpuinfo_max_freq  scaling_available_governors    scaling_setspeed
cpuinfo_min_freq  scaling_cur_freq
[root@intel-canoepass-05 cpufreq]# cat conservative/
down_threshold        sampling_down_factor  up_threshold
freq_step             sampling_rate
ignore_nice_load      sampling_rate_min
[root@intel-canoepass-05 cpufreq]# cat conservative/down_threshold
20
[root@intel-canoepass-05 cpufreq]# echo ondemand > scaling_governor
[root@intel-canoepass-05 cpufreq]# cat ondemand/up_threshold
95
[root@intel-canoepass-05 cpufreq]# echo conservative > scaling_governor
[root@intel-canoepass-05 cpufreq]#

without any issue.  My dmesg (with the printk's) shows

[ 55.331058] cpufreq_set_policy: stopping governor conservative
[ 55.337652] cpufreq_governor_dbs: removing sysfs files for governor conservative
[ 55.346028] cpufreq_set_policy: starting governor ondemand
[ 55.352167] cpufreq_governor_dbs: creating sysfs files for governor ondemand
[ 76.818989] cpufreq_set_policy: stopping governor ondemand
[ 76.825202] cpufreq_governor_dbs: removing sysfs files for governor ondemand
[ 76.833131] cpufreq_set_policy: starting governor conservative
[ 76.839667] cpufreq_governor_dbs: creating sysfs files for governor conservative

There is an already reported LOCKDEP warning in the xfs code that fires at login
so I know LOCKDEP is functional.

Stephen's report as well as the lockup report implies that I should open a file,

-> #1 (&policy->rwsem){+++++.}:
        [<c0359234>] kernfs_fop_open+0x138/0x298
        [<c02fa3f4>] do_dentry_open.isra.12+0x1b0/0x2f0
        [<c02fa604>] finish_open+0x20/0x38
        [<c0308d34>] do_last.isra.37+0x5ac/0xb68
        [<c03093a4>] path_openat+0xb4/0x5d8
        [<c0309bcc>] do_filp_open+0x2c/0x80
        [<c02fb558>] do_sys_open+0x10c/0x1c8
        [<c020f0a0>] ret_fast_syscall+0x0/0x48

and then switch the governor ...

-> #0 (s_active#9){++++..}:
        [<c0357d18>] __kernfs_remove+0x250/0x300
        [<c0358a94>] kernfs_remove_by_name_ns+0x3c/0x84
        [<c035aa78>] remove_files+0x34/0x78
        [<c035aee0>] sysfs_remove_group+0x40/0x98
        [<c05b0560>] cpufreq_governor_dbs+0x4c0/0x6ec
        [<c05abebc>] __cpufreq_governor+0x118/0x200
        [<c05ac0fc>] cpufreq_set_policy+0x158/0x2ac
        [<c05ad5e4>] store_scaling_governor+0x6c/0x94
        [<c05ab210>] store+0x88/0xb8
        [<c035a00c>] sysfs_kf_write+0x4c/0x50
        [<c03594d4>] kernfs_fop_write+0xc0/0x180
        [<c02fc5c8>] vfs_write+0xa0/0x1a8
        [<c02fc9d4>] SyS_write+0x40/0x8c
        [<c020f0a0>] ret_fast_syscall+0x0/0x48

... right?

P.
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cpufreq, store_scaling_governor requires policy->rwsem to be held for duration of changing governors [v2]

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