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Date: Mon, 8 Feb 2016 14:32:13 +0100
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Viresh Kumar <viresh.kumar@...aro.org>
Cc: Rafael Wysocki <rjw@...ysocki.net>,
Juri Lelli <juri.lelli@....com>,
Lists linaro-kernel <linaro-kernel@...ts.linaro.org>,
"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
Saravana Kannan <skannan@...eaurora.org>,
Peter Zijlstra <peterz@...radead.org>,
Michael Turquette <mturquette@...libre.com>,
Steve Muckle <steve.muckle@...aro.org>,
Vincent Guittot <vincent.guittot@...aro.org>,
Morten Rasmussen <morten.rasmussen@....com>,
dietmar.eggemann@....com,
Shilpasri G Bhat <shilpa.bhat@...ux.vnet.ibm.com>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V3 12/13] cpufreq: ondemand: Traverse list of policy_dbs
in update_sampling_rate()
On Mon, Feb 8, 2016 at 12:39 PM, Viresh Kumar <viresh.kumar@...aro.org> wrote:
> Now that we maintain a list of all 'struct policy_dbs_info' for an
> instance of 'struct dbs_data', we can traverse that instead of
> traversing the loop for all online CPUs.
>
> This also solves the circular dependency lockdep reported by Juri
> (and verified by Shilpa) earlier:
>
> ======================================================
> [ INFO: possible circular locking dependency detected ]
> 4.4.0+ #445 Not tainted
> -------------------------------------------------------
> trace.sh/1723 is trying to acquire lock:
> (s_active#48){++++.+}, at: [<c01f78c8>] kernfs_remove_by_name_ns+0x4c/0x94
>
> but task is already holding lock:
> (od_dbs_cdata.mutex){+.+.+.}, at: [<c05824a0>] cpufreq_governor_dbs+0x34/0x5d4
>
> which lock already depends on the new lock.
>
> the existing dependency chain (in reverse order) is:
>
> -> #2 (od_dbs_cdata.mutex){+.+.+.}:
> [<c075b040>] mutex_lock_nested+0x7c/0x434
> [<c05824a0>] cpufreq_governor_dbs+0x34/0x5d4
> [<c0017c10>] return_to_handler+0x0/0x18
>
> -> #1 (&policy->rwsem){+++++.}:
> [<c075ca8c>] down_read+0x58/0x94
> [<c057c244>] show+0x30/0x60
> [<c01f934c>] sysfs_kf_seq_show+0x90/0xfc
> [<c01f7ad8>] kernfs_seq_show+0x34/0x38
> [<c01a22ec>] seq_read+0x1e4/0x4e4
> [<c01f8694>] kernfs_fop_read+0x120/0x1a0
> [<c01794b4>] __vfs_read+0x3c/0xe0
> [<c017a378>] vfs_read+0x98/0x104
> [<c017a434>] SyS_read+0x50/0x90
> [<c000fd40>] ret_fast_syscall+0x0/0x1c
>
> -> #0 (s_active#48){++++.+}:
> [<c008238c>] lock_acquire+0xd4/0x20c
> [<c01f6ae4>] __kernfs_remove+0x288/0x328
> [<c01f78c8>] kernfs_remove_by_name_ns+0x4c/0x94
> [<c01fa024>] remove_files+0x44/0x88
> [<c01fa5a4>] sysfs_remove_group+0x50/0xa4
> [<c058285c>] cpufreq_governor_dbs+0x3f0/0x5d4
> [<c0017c10>] return_to_handler+0x0/0x18
>
> other info that might help us debug this:
>
> Chain exists of:
> s_active#48 --> &policy->rwsem --> od_dbs_cdata.mutex
>
> Possible unsafe locking scenario:
>
> CPU0 CPU1
> ---- ----
> lock(od_dbs_cdata.mutex);
> lock(&policy->rwsem);
> lock(od_dbs_cdata.mutex);
> lock(s_active#48);
>
> *** DEADLOCK ***
>
> 5 locks held by trace.sh/1723:
> #0: (sb_writers#6){.+.+.+}, at: [<c017beb8>] __sb_start_write+0xb4/0xc0
> #1: (&of->mutex){+.+.+.}, at: [<c01f8418>] kernfs_fop_write+0x6c/0x1c8
> #2: (s_active#35){.+.+.+}, at: [<c01f8420>] kernfs_fop_write+0x74/0x1c8
> #3: (cpu_hotplug.lock){++++++}, at: [<c0029e6c>] get_online_cpus+0x48/0xb8
> #4: (od_dbs_cdata.mutex){+.+.+.}, at: [<c05824a0>] cpufreq_governor_dbs+0x34/0x5d4
>
> stack backtrace:
> CPU: 2 PID: 1723 Comm: trace.sh Not tainted 4.4.0+ #445
> Hardware name: ARM-Versatile Express
> [<c001883c>] (unwind_backtrace) from [<c0013f50>] (show_stack+0x20/0x24)
> [<c0013f50>] (show_stack) from [<c044ad90>] (dump_stack+0x80/0xb4)
> [<c044ad90>] (dump_stack) from [<c0128edc>] (print_circular_bug+0x29c/0x2f0)
> [<c0128edc>] (print_circular_bug) from [<c0081708>] (__lock_acquire+0x163c/0x1d74)
> [<c0081708>] (__lock_acquire) from [<c008238c>] (lock_acquire+0xd4/0x20c)
> [<c008238c>] (lock_acquire) from [<c01f6ae4>] (__kernfs_remove+0x288/0x328)
> [<c01f6ae4>] (__kernfs_remove) from [<c01f78c8>] (kernfs_remove_by_name_ns+0x4c/0x94)
> [<c01f78c8>] (kernfs_remove_by_name_ns) from [<c01fa024>] (remove_files+0x44/0x88)
> [<c01fa024>] (remove_files) from [<c01fa5a4>] (sysfs_remove_group+0x50/0xa4)
> [<c01fa5a4>] (sysfs_remove_group) from [<c058285c>] (cpufreq_governor_dbs+0x3f0/0x5d4)
> [<c058285c>] (cpufreq_governor_dbs) from [<c0017c10>] (return_to_handler+0x0/0x18)
>
> Reported-by: Juri Lelli <juri.lelli@....com>
> Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
> ---
> drivers/cpufreq/cpufreq_ondemand.c | 89 ++++++++++----------------------------
> 1 file changed, 22 insertions(+), 67 deletions(-)
>
> diff --git a/drivers/cpufreq/cpufreq_ondemand.c b/drivers/cpufreq/cpufreq_ondemand.c
> index 745290d7f6a2..f72087bc8302 100644
> --- a/drivers/cpufreq/cpufreq_ondemand.c
> +++ b/drivers/cpufreq/cpufreq_ondemand.c
> @@ -224,83 +224,38 @@ static struct dbs_governor od_dbs_gov;
> /**
> * update_sampling_rate - update sampling rate effective immediately if needed.
> * @new_rate: new sampling rate
> - *
> - * If new rate is smaller than the old, simply updating
> - * dbs_tuners_int.sampling_rate might not be appropriate. For example, if the
> - * original sampling_rate was 1 second and the requested new sampling rate is 10
> - * ms because the user needs immediate reaction from ondemand governor, but not
> - * sure if higher frequency will be required or not, then, the governor may
> - * change the sampling rate too late; up to 1 second later. Thus, if we are
> - * reducing the sampling rate, we need to make the new value effective
> - * immediately.
The comment still applies.
Moreover, please extend it to say that this must be called with
dbs_data->mutex held (or it looks racy otherwise).
Thanks,
Rafael
On Mon, Feb 8, 2016 at 12:39 PM, Viresh Kumar <viresh.kumar@...aro.org> wrote:
> Now that we maintain a list of all 'struct policy_dbs_info' for an
> instance of 'struct dbs_data', we can traverse that instead of
> traversing the loop for all online CPUs.
>
> This also solves the circular dependency lockdep reported by Juri
> (and verified by Shilpa) earlier:
>
> ======================================================
> [ INFO: possible circular locking dependency detected ]
> 4.4.0+ #445 Not tainted
> -------------------------------------------------------
> trace.sh/1723 is trying to acquire lock:
> (s_active#48){++++.+}, at: [<c01f78c8>] kernfs_remove_by_name_ns+0x4c/0x94
>
> but task is already holding lock:
> (od_dbs_cdata.mutex){+.+.+.}, at: [<c05824a0>] cpufreq_governor_dbs+0x34/0x5d4
>
> which lock already depends on the new lock.
>
> the existing dependency chain (in reverse order) is:
>
> -> #2 (od_dbs_cdata.mutex){+.+.+.}:
> [<c075b040>] mutex_lock_nested+0x7c/0x434
> [<c05824a0>] cpufreq_governor_dbs+0x34/0x5d4
> [<c0017c10>] return_to_handler+0x0/0x18
>
> -> #1 (&policy->rwsem){+++++.}:
> [<c075ca8c>] down_read+0x58/0x94
> [<c057c244>] show+0x30/0x60
> [<c01f934c>] sysfs_kf_seq_show+0x90/0xfc
> [<c01f7ad8>] kernfs_seq_show+0x34/0x38
> [<c01a22ec>] seq_read+0x1e4/0x4e4
> [<c01f8694>] kernfs_fop_read+0x120/0x1a0
> [<c01794b4>] __vfs_read+0x3c/0xe0
> [<c017a378>] vfs_read+0x98/0x104
> [<c017a434>] SyS_read+0x50/0x90
> [<c000fd40>] ret_fast_syscall+0x0/0x1c
>
> -> #0 (s_active#48){++++.+}:
> [<c008238c>] lock_acquire+0xd4/0x20c
> [<c01f6ae4>] __kernfs_remove+0x288/0x328
> [<c01f78c8>] kernfs_remove_by_name_ns+0x4c/0x94
> [<c01fa024>] remove_files+0x44/0x88
> [<c01fa5a4>] sysfs_remove_group+0x50/0xa4
> [<c058285c>] cpufreq_governor_dbs+0x3f0/0x5d4
> [<c0017c10>] return_to_handler+0x0/0x18
>
> other info that might help us debug this:
>
> Chain exists of:
> s_active#48 --> &policy->rwsem --> od_dbs_cdata.mutex
>
> Possible unsafe locking scenario:
>
> CPU0 CPU1
> ---- ----
> lock(od_dbs_cdata.mutex);
> lock(&policy->rwsem);
> lock(od_dbs_cdata.mutex);
> lock(s_active#48);
>
> *** DEADLOCK ***
>
> 5 locks held by trace.sh/1723:
> #0: (sb_writers#6){.+.+.+}, at: [<c017beb8>] __sb_start_write+0xb4/0xc0
> #1: (&of->mutex){+.+.+.}, at: [<c01f8418>] kernfs_fop_write+0x6c/0x1c8
> #2: (s_active#35){.+.+.+}, at: [<c01f8420>] kernfs_fop_write+0x74/0x1c8
> #3: (cpu_hotplug.lock){++++++}, at: [<c0029e6c>] get_online_cpus+0x48/0xb8
> #4: (od_dbs_cdata.mutex){+.+.+.}, at: [<c05824a0>] cpufreq_governor_dbs+0x34/0x5d4
>
> stack backtrace:
> CPU: 2 PID: 1723 Comm: trace.sh Not tainted 4.4.0+ #445
> Hardware name: ARM-Versatile Express
> [<c001883c>] (unwind_backtrace) from [<c0013f50>] (show_stack+0x20/0x24)
> [<c0013f50>] (show_stack) from [<c044ad90>] (dump_stack+0x80/0xb4)
> [<c044ad90>] (dump_stack) from [<c0128edc>] (print_circular_bug+0x29c/0x2f0)
> [<c0128edc>] (print_circular_bug) from [<c0081708>] (__lock_acquire+0x163c/0x1d74)
> [<c0081708>] (__lock_acquire) from [<c008238c>] (lock_acquire+0xd4/0x20c)
> [<c008238c>] (lock_acquire) from [<c01f6ae4>] (__kernfs_remove+0x288/0x328)
> [<c01f6ae4>] (__kernfs_remove) from [<c01f78c8>] (kernfs_remove_by_name_ns+0x4c/0x94)
> [<c01f78c8>] (kernfs_remove_by_name_ns) from [<c01fa024>] (remove_files+0x44/0x88)
> [<c01fa024>] (remove_files) from [<c01fa5a4>] (sysfs_remove_group+0x50/0xa4)
> [<c01fa5a4>] (sysfs_remove_group) from [<c058285c>] (cpufreq_governor_dbs+0x3f0/0x5d4)
> [<c058285c>] (cpufreq_governor_dbs) from [<c0017c10>] (return_to_handler+0x0/0x18)
>
> Reported-by: Juri Lelli <juri.lelli@....com>
> Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
> ---
> drivers/cpufreq/cpufreq_ondemand.c | 89 ++++++++++----------------------------
> 1 file changed, 22 insertions(+), 67 deletions(-)
>
> diff --git a/drivers/cpufreq/cpufreq_ondemand.c b/drivers/cpufreq/cpufreq_ondemand.c
> index 745290d7f6a2..f72087bc8302 100644
> --- a/drivers/cpufreq/cpufreq_ondemand.c
> +++ b/drivers/cpufreq/cpufreq_ondemand.c
> @@ -224,83 +224,38 @@ static struct dbs_governor od_dbs_gov;
> /**
> * update_sampling_rate - update sampling rate effective immediately if needed.
> * @new_rate: new sampling rate
> - *
> - * If new rate is smaller than the old, simply updating
> - * dbs_tuners_int.sampling_rate might not be appropriate. For example, if the
> - * original sampling_rate was 1 second and the requested new sampling rate is 10
> - * ms because the user needs immediate reaction from ondemand governor, but not
> - * sure if higher frequency will be required or not, then, the governor may
> - * change the sampling rate too late; up to 1 second later. Thus, if we are
> - * reducing the sampling rate, we need to make the new value effective
> - * immediately.
> */
> static void update_sampling_rate(struct dbs_data *dbs_data)
> {
> - struct cpumask cpumask;
> + struct policy_dbs_info *policy_dbs;
> unsigned int new_rate = dbs_data->sampling_rate;
> - int cpu;
>
> /*
> - * Lock governor so that governor start/stop can't execute in parallel.
> + * We are operating under dbs_data->mutex and so the list and its
> + * entries can't be freed concurrently.
> */
> - mutex_lock(&dbs_data_mutex);
> -
> - cpumask_copy(&cpumask, cpu_online_mask);
> -
> - for_each_cpu(cpu, &cpumask) {
> - struct cpufreq_policy *policy;
> - struct od_cpu_dbs_info_s *dbs_info;
> - struct cpu_dbs_info *cdbs;
> - struct policy_dbs_info *policy_dbs;
> -
> - dbs_info = &per_cpu(od_cpu_dbs_info, cpu);
> - cdbs = &dbs_info->cdbs;
> - policy_dbs = cdbs->policy_dbs;
> -
> + list_for_each_entry(policy_dbs, &dbs_data->policy_dbs_list, list) {
> + mutex_lock(&policy_dbs->timer_mutex);
> /*
> - * A valid policy_dbs means governor hasn't stopped or exited
> - * yet.
> + * On 32-bit architectures this may race with the
> + * sample_delay_ns read in dbs_update_util_handler(), but that
> + * really doesn't matter. If the read returns a value that's
> + * too big, the sample will be skipped, but the next invocation
> + * of dbs_update_util_handler() (when the update has been
> + * completed) will take a sample. If the returned value is too
> + * small, the sample will be taken immediately, but that isn't a
> + * problem, as we want the new rate to take effect immediately
> + * anyway.
> + *
> + * If this runs in parallel with dbs_work_handler(), we may end
> + * up overwriting the sample_delay_ns value that it has just
> + * written, but the difference should not be too big and it will
> + * be corrected next time a sample is taken, so it shouldn't be
> + * significant.
> */
> - if (!policy_dbs)
> - continue;
> -
> - policy = policy_dbs->policy;
> -
> - /* clear all CPUs of this policy */
> - cpumask_andnot(&cpumask, &cpumask, policy->cpus);
> -
> - /*
> - * Update sampling rate for CPUs whose policy is governed by
> - * dbs_data. In case of governor_per_policy, only a single
> - * policy will be governed by dbs_data, otherwise there can be
> - * multiple policies that are governed by the same dbs_data.
> - */
> - if (dbs_data == policy_dbs->dbs_data) {
> - mutex_lock(&policy_dbs->timer_mutex);
> - /*
> - * On 32-bit architectures this may race with the
> - * sample_delay_ns read in dbs_update_util_handler(),
> - * but that really doesn't matter. If the read returns
> - * a value that's too big, the sample will be skipped,
> - * but the next invocation of dbs_update_util_handler()
> - * (when the update has been completed) will take a
> - * sample. If the returned value is too small, the
> - * sample will be taken immediately, but that isn't a
> - * problem, as we want the new rate to take effect
> - * immediately anyway.
> - *
> - * If this runs in parallel with dbs_work_handler(), we
> - * may end up overwriting the sample_delay_ns value that
> - * it has just written, but the difference should not be
> - * too big and it will be corrected next time a sample
> - * is taken, so it shouldn't be significant.
> - */
> - gov_update_sample_delay(policy_dbs, new_rate);
> - mutex_unlock(&policy_dbs->timer_mutex);
> - }
> + gov_update_sample_delay(policy_dbs, new_rate);
> + mutex_unlock(&policy_dbs->timer_mutex);
> }
> -
> - mutex_unlock(&dbs_data_mutex);
> }
>
> static bool invalid_up_threshold(struct dbs_data *dbs_data,
> --
> 2.7.1.370.gb2aa7f8
>
> --
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