On 01/24/2011 12:57 PM, Peter Zijlstra wrote:
On Thu, 2011-01-20 at 16:33 -0500, Rik van Riel wrote:The clear_buddies function does not seem to play well with the concept of hierarchical runqueues. In the following tree, task groups are represented by 'G', tasks by 'T', next by 'n' and last by 'l'. (nl) / \ G(nl) G / \ \ T(l) T(n) T This situation can arise when a task is woken up T(n), and the previously running task T(l) is marked last. When clear_buddies is called from either T(l) or T(n), the next and last buddies of the group G(nl) will be cleared. This is not the desired result, since we would like to be able to find the other type of buddy in many cases. This especially a worry when implementing yield_task_fair through the buddy system. The fix is simple: only clear the buddy type that the task itself is indicated to be. As an added bonus, we stop walking up the tree when the buddy has already been cleared or pointed elsewhere. Signed-off-by: Rik van Riel<riel@xxxxxxxxxx> --- kernel/sched_fair.c | 30 +++++++++++++++++++++++------- 1 files changed, 23 insertions(+), 7 deletions(-) diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c index f4ee445..0321473 100644 --- a/kernel/sched_fair.c +++ b/kernel/sched_fair.c @@ -784,19 +784,35 @@ enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags) __enqueue_entity(cfs_rq, se); } -static void __clear_buddies(struct cfs_rq *cfs_rq, struct sched_entity *se) +static void __clear_buddies_last(struct sched_entity *se) { - if (!se || cfs_rq->last == se) - cfs_rq->last = NULL; + for_each_sched_entity(se) { + struct cfs_rq *cfs_rq = cfs_rq_of(se); + if (cfs_rq->last == se) + cfs_rq->last = NULL; + else + break; + } +} - if (!se || cfs_rq->next == se) - cfs_rq->next = NULL; +static void __clear_buddies_next(struct sched_entity *se) +{ + for_each_sched_entity(se) { + struct cfs_rq *cfs_rq = cfs_rq_of(se); + if (cfs_rq->next == se) + cfs_rq->next = NULL; + else + break; + } } static void clear_buddies(struct cfs_rq *cfs_rq, struct sched_entity *se) { - for_each_sched_entity(se) - __clear_buddies(cfs_rq_of(se), se); + if (cfs_rq->last == se) + __clear_buddies_last(se); + + if (cfs_rq->next == se) + __clear_buddies_next(se); }Right, I think this sorta matches with something the Google guys talked about, they wanted to change pick_next_task() no always start from the top but only go up one level when the current level ran out. It looks ok, just sad that we can now have two hierarchy traversals (and 3 with the next patch).
On the other hand, I don't think we'll actually _do_ the hierarchy traversal most of the time, since pick_next_entity calls clear_buddies, every step of the way down the tree. A hierarchy traversal will only be done if a task already has one type of buddy set, and then gets another type of buddy set, before it is rescheduled. Eg. a task can have ->last set and then call yield, causing the ->yield buddy to get pointed at itself. When doing that, it will walk up the tree, clearing ->last. I suspect that with this patch, we'll end up doing less tree traversal than before. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
