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authorPaul Mackerras <paulus@samba.org>2009-05-22 14:27:22 +1000
committerIngo Molnar <mingo@elte.hu>2009-05-22 12:18:20 +0200
commit564c2b210add41df9a3a5aaa365c1d97cff6110d (patch)
tree2fe54df816abfe8c6010d277c79fbe18aa5c4082 /include/linux
parenta63eaf34ae60bdb067a354cc8def2e8f4a01f5f4 (diff)
perf_counter: Optimize context switch between identical inherited contexts
When monitoring a process and its descendants with a set of inherited counters, we can often get the situation in a context switch where both the old (outgoing) and new (incoming) process have the same set of counters, and their values are ultimately going to be added together. In that situation it doesn't matter which set of counters are used to count the activity for the new process, so there is really no need to go through the process of reading the hardware counters and updating the old task's counters and then setting up the PMU for the new task. This optimizes the context switch in this situation. Instead of scheduling out the perf_counter_context for the old task and scheduling in the new context, we simply transfer the old context to the new task and keep using it without interruption. The new context gets transferred to the old task. This means that both tasks still have a valid perf_counter_context, so no special case is introduced when the old task gets scheduled in again, either on this CPU or another CPU. The equivalence of contexts is detected by keeping a pointer in each cloned context pointing to the context it was cloned from. To cope with the situation where a context is changed by adding or removing counters after it has been cloned, we also keep a generation number on each context which is incremented every time a context is changed. When a context is cloned we take a copy of the parent's generation number, and two cloned contexts are equivalent only if they have the same parent and the same generation number. In order that the parent context pointer remains valid (and is not reused), we increment the parent context's reference count for each context cloned from it. Since we don't have individual fds for the counters in a cloned context, the only thing that can make two clones of a given parent different after they have been cloned is enabling or disabling all counters with prctl. To account for this, we keep a count of the number of enabled counters in each context. Two contexts must have the same number of enabled counters to be considered equivalent. Here are some measurements of the context switch time as measured with the lat_ctx benchmark from lmbench, comparing the times obtained with and without this patch series: -----Unmodified----- With this patch series Counters: none 2 HW 4H+4S none 2 HW 4H+4S 2 processes: Average 3.44 6.45 11.24 3.12 3.39 3.60 St dev 0.04 0.04 0.13 0.05 0.17 0.19 8 processes: Average 6.45 8.79 14.00 5.57 6.23 7.57 St dev 1.27 1.04 0.88 1.42 1.46 1.42 32 processes: Average 5.56 8.43 13.78 5.28 5.55 7.15 St dev 0.41 0.47 0.53 0.54 0.57 0.81 The numbers are the mean and standard deviation of 20 runs of lat_ctx. The "none" columns are lat_ctx run directly without any counters. The "2 HW" columns are with lat_ctx run under perfstat, counting cycles and instructions. The "4H+4S" columns are lat_ctx run under perfstat with 4 hardware counters and 4 software counters (cycles, instructions, cache references, cache misses, task clock, context switch, cpu migrations, and page faults). [ Impact: performance optimization of counter context-switches ] Signed-off-by: Paul Mackerras <paulus@samba.org> Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com> Cc: Marcelo Tosatti <mtosatti@redhat.com> Cc: Arnaldo Carvalho de Melo <acme@redhat.com> LKML-Reference: <18966.10666.517218.332164@cargo.ozlabs.ibm.com> Signed-off-by: Ingo Molnar <mingo@elte.hu>
Diffstat (limited to 'include/linux')
-rw-r--r--include/linux/perf_counter.h12
1 files changed, 11 insertions, 1 deletions
diff --git a/include/linux/perf_counter.h b/include/linux/perf_counter.h
index 07130900546..4cae01a5045 100644
--- a/include/linux/perf_counter.h
+++ b/include/linux/perf_counter.h
@@ -513,6 +513,7 @@ struct perf_counter_context {
struct list_head event_list;
int nr_counters;
int nr_active;
+ int nr_enabled;
int is_active;
atomic_t refcount;
struct task_struct *task;
@@ -522,6 +523,14 @@ struct perf_counter_context {
*/
u64 time;
u64 timestamp;
+
+ /*
+ * These fields let us detect when two contexts have both
+ * been cloned (inherited) from a common ancestor.
+ */
+ struct perf_counter_context *parent_ctx;
+ u32 parent_gen;
+ u32 generation;
};
/**
@@ -552,7 +561,8 @@ extern int perf_max_counters;
extern const struct pmu *hw_perf_counter_init(struct perf_counter *counter);
extern void perf_counter_task_sched_in(struct task_struct *task, int cpu);
-extern void perf_counter_task_sched_out(struct task_struct *task, int cpu);
+extern void perf_counter_task_sched_out(struct task_struct *task,
+ struct task_struct *next, int cpu);
extern void perf_counter_task_tick(struct task_struct *task, int cpu);
extern void perf_counter_init_task(struct task_struct *child);
extern void perf_counter_exit_task(struct task_struct *child);