The following commit has been merged into the perf/core branch of tip: Commit-ID: 217c2a633ebb36f1cc6d249f4ef2e4a809d46818 Gitweb: https://git.kernel.org/tip/217c2a633ebb36f1cc6d249f4ef2e4a809d46818 Author: Kan Liang <kan.liang@xxxxxxxxxxxxxxx> AuthorDate: Fri, 03 Jul 2020 05:49:22 -07:00 Committer: Peter Zijlstra <peterz@xxxxxxxxxxxxx> CommitterDate: Wed, 08 Jul 2020 11:38:55 +02:00 perf/core: Use kmem_cache to allocate the PMU specific data Currently, the PMU specific data task_ctx_data is allocated by the function kzalloc() in the perf generic code. When there is no specific alignment requirement for the task_ctx_data, the method works well for now. However, there will be a problem once a specific alignment requirement is introduced in future features, e.g., the Architecture LBR XSAVE feature requires 64-byte alignment. If the specific alignment requirement is not fulfilled, the XSAVE family of instructions will fail to save/restore the xstate to/from the task_ctx_data. The function kzalloc() itself only guarantees a natural alignment. A new method to allocate the task_ctx_data has to be introduced, which has to meet the requirements as below: - must be a generic method can be used by different architectures, because the allocation of the task_ctx_data is implemented in the perf generic code; - must be an alignment-guarantee method (The alignment requirement is not changed after the boot); - must be able to allocate/free a buffer (smaller than a page size) dynamically; - should not cause extra CPU overhead or space overhead. Several options were considered as below: - One option is to allocate a larger buffer for task_ctx_data. E.g., ptr = kmalloc(size + alignment, GFP_KERNEL); ptr &= ~(alignment - 1); This option causes space overhead. - Another option is to allocate the task_ctx_data in the PMU specific code. To do so, several function pointers have to be added. As a result, both the generic structure and the PMU specific structure will become bigger. Besides, extra function calls are added when allocating/freeing the buffer. This option will increase both the space overhead and CPU overhead. - The third option is to use a kmem_cache to allocate a buffer for the task_ctx_data. The kmem_cache can be created with a specific alignment requirement by the PMU at boot time. A new pointer for kmem_cache has to be added in the generic struct pmu, which would be used to dynamically allocate a buffer for the task_ctx_data at run time. Although the new pointer is added to the struct pmu, the existing variable task_ctx_size is not required anymore. The size of the generic structure is kept the same. The third option which meets all the aforementioned requirements is used to replace kzalloc() for the PMU specific data allocation. A later patch will remove the kzalloc() method and the related variables. Signed-off-by: Kan Liang <kan.liang@xxxxxxxxxxxxxxx> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> Link: https://lkml.kernel.org/r/1593780569-62993-17-git-send-email-kan.liang@xxxxxxxxxxxxxxx --- include/linux/perf_event.h | 5 +++++ kernel/events/core.c | 8 +++++++- 2 files changed, 12 insertions(+), 1 deletion(-) diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h index 46fe5cf..09915ae 100644 --- a/include/linux/perf_event.h +++ b/include/linux/perf_event.h @@ -425,6 +425,11 @@ struct pmu { size_t task_ctx_size; /* + * Kmem cache of PMU specific data + */ + struct kmem_cache *task_ctx_cache; + + /* * PMU specific parts of task perf event context (i.e. ctx->task_ctx_data) * can be synchronized using this function. See Intel LBR callstack support * implementation and Perf core context switch handling callbacks for usage diff --git a/kernel/events/core.c b/kernel/events/core.c index 7509040..30d9b31 100644 --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -1240,12 +1240,18 @@ static void get_ctx(struct perf_event_context *ctx) static void *alloc_task_ctx_data(struct pmu *pmu) { + if (pmu->task_ctx_cache) + return kmem_cache_zalloc(pmu->task_ctx_cache, GFP_KERNEL); + return kzalloc(pmu->task_ctx_size, GFP_KERNEL); } static void free_task_ctx_data(struct pmu *pmu, void *task_ctx_data) { - kfree(task_ctx_data); + if (pmu->task_ctx_cache && task_ctx_data) + kmem_cache_free(pmu->task_ctx_cache, task_ctx_data); + else + kfree(task_ctx_data); } static void free_ctx(struct rcu_head *head)
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