Message ID | 1436929315-28520-8-git-send-email-sukadev@linux.vnet.ibm.com (mailing list archive) |
---|---|
State | Superseded |
Headers | show |
On Tue, Jul 14, 2015 at 08:01:54PM -0700, Sukadev Bhattiprolu wrote: > +/* > + * Use the transaction interface to read the group of events in @leader. > + * PMUs like the 24x7 counters in Power, can use this to queue the events > + * in the ->read() operation and perform the actual read in ->commit_txn. > + * > + * Other PMUs can ignore the ->start_txn and ->commit_txn and read each > + * PMU directly in the ->read() operation. > + */ > +static int perf_event_read_group(struct perf_event *leader) > +{ > + int ret; > + struct perf_event *sub; > + struct pmu *pmu; > + > + pmu = leader->pmu; > + > + pmu->start_txn(pmu, PERF_PMU_TXN_READ); > + > + perf_event_read(leader); There should be a lockdep assert with that list iteration. > + list_for_each_entry(sub, &leader->sibling_list, group_entry) > + perf_event_read(sub); > + > + ret = pmu->commit_txn(pmu); > + > + return ret; > +}
Peter Zijlstra [peterz@infradead.org] wrote: | On Tue, Jul 14, 2015 at 08:01:54PM -0700, Sukadev Bhattiprolu wrote: | > +/* | > + * Use the transaction interface to read the group of events in @leader. | > + * PMUs like the 24x7 counters in Power, can use this to queue the events | > + * in the ->read() operation and perform the actual read in ->commit_txn. | > + * | > + * Other PMUs can ignore the ->start_txn and ->commit_txn and read each | > + * PMU directly in the ->read() operation. | > + */ | > +static int perf_event_read_group(struct perf_event *leader) | > +{ | > + int ret; | > + struct perf_event *sub; | > + struct pmu *pmu; | > + | > + pmu = leader->pmu; | > + | > + pmu->start_txn(pmu, PERF_PMU_TXN_READ); | > + | > + perf_event_read(leader); | | There should be a lockdep assert with that list iteration. | | > + list_for_each_entry(sub, &leader->sibling_list, group_entry) | > + perf_event_read(sub); | > + | > + ret = pmu->commit_txn(pmu); Peter, I have a situation :-) We are trying to use the following interface: start_txn(pmu, PERF_PMU_TXN_READ); perf_event_read(leader); list_for_each(sibling, &leader->sibling_list, group_entry) perf_event_read(sibling) pmu->commit_txn(pmu); with the idea that the PMU driver would save the type of transaction in ->start_txn() and use in ->read() and ->commit_txn(). But since ->start_txn() and the ->read() operations could happen on different CPUs (perf_event_read() uses the event->oncpu to schedule a call), the PMU driver cannot use a per-cpu variable to save the state in ->start_txn(). I tried using a pmu-wide global, but that would also need us to hold a mutex to serialize access to that global. The problem is ->start_txn() can be called from an interrupt context for the TXN_ADD transactions (I got the following backtrace during testing) mutex_lock_nested+0x504/0x520 (unreliable) h_24x7_event_start_txn+0x3c/0xd0 group_sched_in+0x70/0x230 ctx_sched_in.isra.63+0x150/0x230 __perf_install_in_context+0x1c8/0x1e0 remote_function+0x7c/0xa0 flush_smp_call_function_queue+0xb0/0x1d0 smp_ipi_demux+0x88/0xf0 icp_hv_ipi_action+0x54/0xc0 handle_irq_event_percpu+0x98/0x2b0 handle_percpu_irq+0x7c/0xc0 generic_handle_irq+0x4c/0x80 __do_irq+0x7c/0x190 call_do_irq+0x14/0x24 do_IRQ+0x8c/0x100 hardware_interrupt_common+0x168/0x180 --- interrupt: 501 at .plpar_hcall_norets+0x14/0x20 Basically stuck trying to save the txn type in ->start_txn() and retrieve in ->read(). Couple of options I can think of are: - having ->start_txn() return a handle that should then be passed in with ->read() (yuck) and ->commit_txn(). - serialize the READ transaction for the PMU in perf_event_read_group() with a new pmu->txn_mutex: mutex_lock(&pmu->txn_mutex); pmu->start_txn() list_for_each_entry(sub, &leader->sibling_list, group_entry) perf_event_read(sub); ret = pmu->commit_txn(pmu); mutex_unlock(&pmu->txn_mutex); such serialization would be ok with 24x7 counters (they are system wide counters anyway) We could maybe skip the mutex for PMUs that don't implement TXN_READ interface. or is there better way? Sukadev
On Tue, Jul 21, 2015 at 06:50:45PM -0700, Sukadev Bhattiprolu wrote: > We are trying to use the following interface: > > start_txn(pmu, PERF_PMU_TXN_READ); > > perf_event_read(leader); > list_for_each(sibling, &leader->sibling_list, group_entry) > perf_event_read(sibling) > > pmu->commit_txn(pmu); > > with the idea that the PMU driver would save the type of transaction in > ->start_txn() and use in ->read() and ->commit_txn(). > > But since ->start_txn() and the ->read() operations could happen on different > CPUs (perf_event_read() uses the event->oncpu to schedule a call), the PMU > driver cannot use a per-cpu variable to save the state in ->start_txn(). > or is there better way? I've not woken up yet, and not actually fully read the email, but can you stuff the entire above chunk inside the IPI? I think you could then actually optimize __perf_event_read() as well, because all these events should be on the same context, so no point in calling update_*time*() for every event or so.
diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h index 44bf05f..da307ad 100644 --- a/include/linux/perf_event.h +++ b/include/linux/perf_event.h @@ -169,6 +169,7 @@ struct perf_event; #define PERF_EVENT_TXN 0x1 #define PERF_PMU_TXN_ADD 0x1 /* txn to add/schedule event on PMU */ +#define PERF_PMU_TXN_READ 0x2 /* txn to read event group from PMU */ /** * pmu::capabilities flags diff --git a/kernel/events/core.c b/kernel/events/core.c index a83d45c..2ea06c4 100644 --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -3763,6 +3763,33 @@ static void orphans_remove_work(struct work_struct *work) put_ctx(ctx); } +/* + * Use the transaction interface to read the group of events in @leader. + * PMUs like the 24x7 counters in Power, can use this to queue the events + * in the ->read() operation and perform the actual read in ->commit_txn. + * + * Other PMUs can ignore the ->start_txn and ->commit_txn and read each + * PMU directly in the ->read() operation. + */ +static int perf_event_read_group(struct perf_event *leader) +{ + int ret; + struct perf_event *sub; + struct pmu *pmu; + + pmu = leader->pmu; + + pmu->start_txn(pmu, PERF_PMU_TXN_READ); + + perf_event_read(leader); + list_for_each_entry(sub, &leader->sibling_list, group_entry) + perf_event_read(sub); + + ret = pmu->commit_txn(pmu); + + return ret; +} + u64 perf_event_read_value(struct perf_event *event, u64 *enabled, u64 *running) { u64 total = 0; @@ -3792,7 +3819,11 @@ static int perf_read_group(struct perf_event *event, lockdep_assert_held(&ctx->mutex); - count = perf_event_read_value(leader, &enabled, &running); + ret = perf_event_read_group(leader); + if (ret) + return ret; + + count = perf_event_compute(leader, &enabled, &running); values[n++] = 1 + leader->nr_siblings; if (read_format & PERF_FORMAT_TOTAL_TIME_ENABLED) @@ -3813,7 +3844,7 @@ static int perf_read_group(struct perf_event *event, list_for_each_entry(sub, &leader->sibling_list, group_entry) { n = 0; - values[n++] = perf_event_read_value(sub, &enabled, &running); + values[n++] = perf_event_compute(sub, &enabled, &running); if (read_format & PERF_FORMAT_ID) values[n++] = primary_event_id(sub);
Define a new PERF_PMU_TXN_READ interface to read a group of counters at once. Note that we use this interface with all PMUs. PMUs that implement this interface use the ->read() operation to _queue_ the counters to be read and use ->commit_txn() to actually read all the queued counters at once. PMUs that don't implement PERF_PMU_TXN_READ ignore ->start_txn() and ->commit_txn() and continue to read counters one at a time. Thanks to input from Peter Zijlstra. Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com> --- include/linux/perf_event.h | 1 + kernel/events/core.c | 35 +++++++++++++++++++++++++++++++++-- 2 files changed, 34 insertions(+), 2 deletions(-)