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| author | Borislav Petkov <bp@alien8.de> | 2011-03-27 17:57:13 +0200 |
|---|---|---|
| committer | Ingo Molnar <mingo@elte.hu> | 2011-03-31 13:00:35 +0200 |
| commit | e2495b577324938f0209b4f895c5f205c7e47854 (patch) | |
| tree | 790da0f9afd36eb3bdb83c9dd0e885ed81c30096 /Documentation | |
| parent | a51e91981870d013fcfcc08b0117997edbcbc7a7 (diff) | |
| download | lwn-e2495b577324938f0209b4f895c5f205c7e47854.tar.gz lwn-e2495b577324938f0209b4f895c5f205c7e47854.zip | |
sched, doc: Beef up load balancing description
Correct all function names pertaining to load balancing and explain
shortly how load balancing is performed.
Signed-off-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1301241433-3790-1-git-send-email-bp@alien8.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Diffstat (limited to 'Documentation')
| -rw-r--r-- | Documentation/scheduler/sched-domains.txt | 32 |
1 files changed, 23 insertions, 9 deletions
diff --git a/Documentation/scheduler/sched-domains.txt b/Documentation/scheduler/sched-domains.txt index 373ceacc367e..b7ee379b651b 100644 --- a/Documentation/scheduler/sched-domains.txt +++ b/Documentation/scheduler/sched-domains.txt @@ -1,8 +1,7 @@ -Each CPU has a "base" scheduling domain (struct sched_domain). These are -accessed via cpu_sched_domain(i) and this_sched_domain() macros. The domain +Each CPU has a "base" scheduling domain (struct sched_domain). The domain hierarchy is built from these base domains via the ->parent pointer. ->parent -MUST be NULL terminated, and domain structures should be per-CPU as they -are locklessly updated. +MUST be NULL terminated, and domain structures should be per-CPU as they are +locklessly updated. Each scheduling domain spans a number of CPUs (stored in the ->span field). A domain's span MUST be a superset of it child's span (this restriction could @@ -26,11 +25,26 @@ is treated as one entity. The load of a group is defined as the sum of the load of each of its member CPUs, and only when the load of a group becomes out of balance are tasks moved between groups. -In kernel/sched.c, rebalance_tick is run periodically on each CPU. This -function takes its CPU's base sched domain and checks to see if has reached -its rebalance interval. If so, then it will run load_balance on that domain. -rebalance_tick then checks the parent sched_domain (if it exists), and the -parent of the parent and so forth. +In kernel/sched.c, trigger_load_balance() is run periodically on each CPU +through scheduler_tick(). It raises a softirq after the next regularly scheduled +rebalancing event for the current runqueue has arrived. The actual load +balancing workhorse, run_rebalance_domains()->rebalance_domains(), is then run +in softirq context (SCHED_SOFTIRQ). + +The latter function takes two arguments: the current CPU and whether it was idle +at the time the scheduler_tick() happened and iterates over all sched domains +our CPU is on, starting from its base domain and going up the ->parent chain. +While doing that, it checks to see if the current domain has exhausted its +rebalance interval. If so, it runs load_balance() on that domain. It then checks +the parent sched_domain (if it exists), and the parent of the parent and so +forth. + +Initially, load_balance() finds the busiest group in the current sched domain. +If it succeeds, it looks for the busiest runqueue of all the CPUs' runqueues in +that group. If it manages to find such a runqueue, it locks both our initial +CPU's runqueue and the newly found busiest one and starts moving tasks from it +to our runqueue. The exact number of tasks amounts to an imbalance previously +computed while iterating over this sched domain's groups. *** Implementing sched domains *** The "base" domain will "span" the first level of the hierarchy. In the case |