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Message-ID: <AANLkTimBQqq3iQaqXfchXoA+A5m_VdnS=_Q96wWAm1Tm@mail.gmail.com>
Date: Thu, 9 Dec 2010 01:15:25 +0100
From: Stephane Eranian <eranian@...gle.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: linux-kernel@...r.kernel.org, mingo@...e.hu, paulus@...ba.org,
davem@...emloft.net, fweisbec@...il.com,
perfmon2-devel@...ts.sf.net, eranian@...il.com,
robert.richter@....com, acme@...hat.com, lizf@...fujitsu.com
Subject: Re: [PATCH 4/5] perf_events: add cgroup support (v6)
On Wed, Dec 1, 2010 at 3:00 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Tue, 2010-11-30 at 19:20 +0200, Stephane Eranian wrote:
>
>> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
>> index 66a416b..1c8bee8 100644
>> --- a/kernel/cgroup.c
>> +++ b/kernel/cgroup.c
>> @@ -4790,6 +4790,29 @@ css_get_next(struct cgroup_subsys *ss, int id,
>> return ret;
>> }
>>
>> +/*
>> + * get corresponding css from file open on cgroupfs directory
>> + */
>> +struct cgroup_subsys_state *cgroup_css_from_dir(struct file *f, int id)
>> +{
>> + struct cgroup *cgrp;
>> + struct inode *inode;
>> + struct cgroup_subsys_state *css;
>> +
>> + inode = f->f_dentry->d_inode;
>> + /* check in cgroup filesystem dir */
>> + if (inode->i_op != &cgroup_dir_inode_operations)
>> + return ERR_PTR(-EBADF);
>> +
>> + if (id < 0 || id >= CGROUP_SUBSYS_COUNT)
>> + return ERR_PTR(-EINVAL);
>> +
>> + /* get cgroup */
>> + cgrp = __d_cgrp(f->f_dentry);
>> + css = cgrp->subsys[id];
>> + return css ? css : ERR_PTR(-ENOENT);
>> +}
>
> Since this paradigm was already in use it surprises me you have to add
> this function.. ?
>
Well, I could not find one. If anybody knows of one, I'll be able to
check it out.
>> +#ifdef CONFIG_PERF_CGROUPS
>> +static inline struct perf_cgroup *
>> +perf_cgroup_from_task(struct task_struct *task)
>> +{
>> + if (!task)
>> + return NULL;
>> + return container_of(task_subsys_state(task, perf_subsys_id),
>> + struct perf_cgroup, css);
>> +}
>
> Wouldn't it be nicer if the caller ensured to not call it for !task?
>
>
>> +static struct perf_cgroup *perf_get_cgroup(int fd)
>> +{
>> + struct cgroup_subsys_state *css;
>> + struct file *file;
>> + int fput_needed;
>> +
>> + file = fget_light(fd, &fput_needed);
>> + if (!file)
>> + return ERR_PTR(-EBADF);
>> +
>> + css = cgroup_css_from_dir(file, perf_subsys_id);
>> + if (!IS_ERR(css))
>> + css_get(css);
>> +
>> + fput_light(file, fput_needed);
>> +
>> + return container_of(css, struct perf_cgroup, css);
>> +}
>> +
>> +static inline void perf_put_cgroup(struct perf_event *event)
>> +{
>> + if (event->cgrp)
>> + css_put(&event->cgrp->css);
>> +}
>
> Bit asymmetric, you get a perf_cgroup, but you put a perf_event.
>
Ok, I made this symmetrical now.
>
>> +static inline void __update_css_time(struct perf_cgroup *cgrp)
>> +{
>> + struct perf_cgroup_info *t;
>> + u64 now;
>> + int cpu = smp_processor_id();
>> +
>> + if (!cgrp)
>> + return;
>> +
>> + now = perf_clock();
>> +
>> + t = per_cpu_ptr(cgrp->info, cpu);
>> +
>> + t->time += now - t->timestamp;
>> + t->timestamp = now;
>> +}
>
> Most callers seem to already check for !cgrp, make that all and avoid
> the second conditional?
>
Done.
>> +/*
>> + * called from perf_event_ask_sched_out() conditional to jump label
>> + */
>> +void
>> +perf_cgroup_switch(struct task_struct *task, struct task_struct *next)
>> +{
>> + struct perf_cgroup *cgrp_out = perf_cgroup_from_task(task);
>> + struct perf_cgroup *cgrp_in = perf_cgroup_from_task(next);
>> + struct perf_cpu_context *cpuctx;
>> + struct pmu *pmu;
>> + /*
>> + * if task is DEAD, then css_out is irrelevant, it has
>> + * been changed to init_css in cgroup_exit() from do_exit().
>> + * Furthermore, perf_cgroup_exit_task(), has scheduled out
>> + * all css constrained events, only unconstrained events
>> + * remain. Therefore we need to reschedule based on css_in.
>> + */
>> + if (task->state != TASK_DEAD && cgrp_out == cgrp_in)
>> + return;
>> +
>> + rcu_read_lock();
>> +
>> + list_for_each_entry_rcu(pmu, &pmus, entry) {
>> +
>> + cpuctx = get_cpu_ptr(pmu->pmu_cpu_context);
>> +
>> + perf_pmu_disable(cpuctx->ctx.pmu);
>> +
>> + /*
>> + * perf_cgroup_events says at least one
>> + * context on this CPU has cgroup events.
>> + *
>> + * ctx->nr_cgroups reports the number of cgroup
>> + * events for a context. Given there can be multiple
>> + * PMUs, there can be multiple contexts.
>> + */
>> + if (cpuctx->ctx.nr_cgroups > 0) {
>> + /*
>> + * schedule out everything we have
>> + * task == DEAD: only unconstrained events
>> + * task != DEAD: css constrained + unconstrained events
>> + *
>> + * We kick out all events (even if unconstrained)
>> + * to allow the constrained events to be scheduled
>> + * based on their position in the event list (fairness)
>> + */
>> + cpu_ctx_sched_out(cpuctx, EVENT_ALL);
>> + /*
>> + * reschedule css_in constrained + unconstrained events
>> + */
>> + cpu_ctx_sched_in(cpuctx, EVENT_ALL, next, 1);
>> + }
>> +
>> + perf_pmu_enable(cpuctx->ctx.pmu);
>
> Do you leak a preemption count here? No matching put_cpu_ptr().
>
> Since we're in the middle of a context switch, preemption is already
> disabled and it might be best to use this_cpu_ptr() instead of
> get_cpu_ptr(). That avoids the preemption bits.
>
Done.
>> +static inline void
>> +perf_cgroup_exit_task(struct task_struct *task)
>> +{
>> + struct perf_cpu_context *cpuctx;
>> + struct pmu *pmu;
>> + unsigned long flags;
>> +
>> + local_irq_save(flags);
>> +
>> + rcu_read_lock();
>> +
>> + list_for_each_entry_rcu(pmu, &pmus, entry) {
>> +
>> + cpuctx = get_cpu_ptr(pmu->pmu_cpu_context);
>> +
>> + perf_pmu_disable(cpuctx->ctx.pmu);
>> +
>> + if (cpuctx->ctx.nr_cgroups > 0) {
>> + /*
>> + * task is going to be detached from css.
>> + * We cannot keep a reference on the css
>> + * as it may disappear before we get to
>> + * perf_cgroup_switch(). Thus, we remove
>> + * all css constrained events.
>> + *
>> + * We do this by scheduling out everything
>> + * we have, and then only rescheduling only
>> + * the unconstrained events. Those can keep
>> + * on counting.
>> + *
>> + * We re-examine the situation in the final
>> + * perf_cgroup_switch() call for this task
>> + * once we know the next task.
>> + */
>> + cpu_ctx_sched_out(cpuctx, EVENT_ALL);
>> + /*
>> + * task = NULL causes perf_cgroup_match()
>> + * to match only unconstrained events
>> + */
>> + cpu_ctx_sched_in(cpuctx, EVENT_ALL, NULL, 1);
>> + }
>> +
>> + perf_pmu_enable(cpuctx->ctx.pmu);
>
> Another preemption leak?
>
Done.
>
>> @@ -246,6 +581,10 @@ static void update_context_time(struct perf_event_context *ctx)
>> static u64 perf_event_time(struct perf_event *event)
>> {
>> struct perf_event_context *ctx = event->ctx;
>> +
>> + if (is_cgroup_event(event))
>> + return perf_cgroup_event_css_time(event);
>> +
>> return ctx ? ctx->time : 0;
>> }
>>
>> @@ -261,8 +600,10 @@ static void update_event_times(struct perf_event *event)
>> event->group_leader->state < PERF_EVENT_STATE_INACTIVE)
>> return;
>>
>> - if (ctx->is_active)
>> - run_end = perf_event_time(event);
>> + if (is_cgroup_event(event))
>> + run_end = perf_cgroup_event_css_time(event);
>> + else if (ctx->is_active)
>> + run_end = ctx->time;
>> else
>> run_end = event->tstamp_stopped;
>
> So I guess the difference is that we want perf_cgroup_event_css_time()
> even when !active?
>
The difference is in the way time_enabled in accounted for in cgroup mode.
time_enabled represent the time the event is enabled AND the monitored
threads were active of the monitored CPU. Thus it is independent of the
state of the context. A context may have cgroup and non-cgroup events
attached to it. I have added a comment to explain that.
>> @@ -322,6 +663,17 @@ list_add_event(struct perf_event *event, struct perf_event_context *ctx)
>> list_add_tail(&event->group_entry, list);
>> }
>>
>> + if (is_cgroup_event(event)) {
>> + ctx->nr_cgroups++;
>> + /*
>> + * one more event:
>> + * - that has cgroup constraint on event->cpu
>> + * - that may need work on context switch
>> + */
>> + atomic_inc(&per_cpu(perf_cgroup_events, event->cpu));
>> + jump_label_inc(&perf_sched_events);
>> + }
>
> Ah, I guess this is why you're still using atomics, since another cpu
> can install the counters on the target cpu,. ok I guess that makes
> sense.
>
YES!
>> - event->shadow_ctx_time = tstamp - ctx->timestamp;
>> + /*
>> + * use the correct time source for the time snapshot
>> + *
>> + * We could get by without this by leveraging the
>> + * fact that to get to this function, the caller
>> + * has most likely already called update_context_time()
>> + * and update_css_time_xx() and thus both timestamp
>> + * are identical (or very close). Given that tstamp is,
>> + * already adjusted for cgroup, we could say that:
>> + * tstamp - ctx->timestamp
>> + * is equivalent to
>> + * tstamp - cgrp->timestamp.
>> + *
>> + * Then, in perf_output_read(), the calculation would
>> + * work with no changes because:
>> + * - event is guaranteed scheduled in
>> + * - no scheduled out in between
>> + * - thus the timestamp would be the same
>> + *
>> + * But this is a bit hairy.
>> + *
>> + * So instead, we have an explicit cgroup call to remain
>> + * within the time time source all along. We believe it
>> + * is cleaner and simpler to understand.
>> + */
>> + if (is_cgroup_event(event))
>> + perf_cgroup_set_shadow_time(event, tstamp);
>> + else
>> + event->shadow_ctx_time = tstamp - ctx->timestamp;
>
> How about we make perf_set_shadow_time() and hide all this in there?
>
>
Done.
>> @@ -5289,6 +5719,7 @@ unlock:
>> static struct perf_event *
>> perf_event_alloc(struct perf_event_attr *attr, int cpu,
>> struct task_struct *task,
>> + int cgrp_fd, int flags,
>> struct perf_event *group_leader,
>> struct perf_event *parent_event,
>> perf_overflow_handler_t overflow_handler)
>> @@ -5302,6 +5733,14 @@ perf_event_alloc(struct perf_event_attr *attr, int cpu,
>> if (!event)
>> return ERR_PTR(-ENOMEM);
>>
>> + if (flags & PERF_FLAG_PID_CGROUP) {
>> + err = perf_connect_cgroup(cgrp_fd, event, attr, group_leader);
>> + if (err) {
>> + kfree(event);
>> + return ERR_PTR(err);
>> + }
>> + }
>> +
>> /*
>> * Single events are their own group leaders, with an
>> * empty sibling list:
>
>
> Hrm,. that isn't particularly pretty,.. why do we have to do this in
> perf_event_alloc()? Can't we do this in the syscall after
> perf_event_alloc() returns?
>
Done.
Will be posting an updated version soon.
I also realized, I need to check how cgroup is handled for the SW events.
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