From: Mathieu Desnoyers mathieu.desnoyers@efficios.com
When a process reduces its number of threads or clears bits in its CPU affinity mask, the mm_cid allocation should eventually converge towards smaller values.
However, the change introduced by:
commit 7e019dcc470f ("sched: Improve cache locality of RSEQ concurrency IDs for intermittent workloads")
adds a per-mm/CPU recent_cid which is never unset unless a thread migrates.
This is a tradeoff between:
A) Preserving cache locality after a transition from many threads to few threads, or after reducing the hamming weight of the allowed CPU mask.
B) Making the mm_cid upper bounds wrt nr threads and allowed CPU mask easy to document and understand.
C) Allowing applications to eventually react to mm_cid compaction after reduction of the nr threads or allowed CPU mask, making the tracking of mm_cid compaction easier by shrinking it back towards 0 or not.
D) Making sure applications that periodically reduce and then increase again the nr threads or allowed CPU mask still benefit from good cache locality with mm_cid.
Introduce the following changes:
* After shrinking the number of threads or reducing the number of allowed CPUs, reduce the value of max_nr_cid so expansion of CID allocation will preserve cache locality if the number of threads or allowed CPUs increase again.
* Only re-use a recent_cid if it is within the max_nr_cid upper bound, else find the first available CID.
Fixes: 7e019dcc470f ("sched: Improve cache locality of RSEQ concurrency IDs for intermittent workloads") Cc: Peter Zijlstra (Intel) peterz@infradead.org Cc: Marco Elver elver@google.com Cc: Ingo Molnar mingo@kernel.org Tested-by: Gabriele Monaco gmonaco@redhat.com Signed-off-by: Mathieu Desnoyers mathieu.desnoyers@efficios.com Signed-off-by: Gabriele Monaco gmonaco@redhat.com --- include/linux/mm_types.h | 7 ++++--- kernel/sched/sched.h | 25 ++++++++++++++++++++++--- 2 files changed, 26 insertions(+), 6 deletions(-)
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h index 8a76a1c09234..16076e70a6b9 100644 --- a/include/linux/mm_types.h +++ b/include/linux/mm_types.h @@ -837,10 +837,11 @@ struct mm_struct { */ unsigned int nr_cpus_allowed; /** - * @max_nr_cid: Maximum number of concurrency IDs allocated. + * @max_nr_cid: Maximum number of allowed concurrency + * IDs allocated. * - * Track the highest number of concurrency IDs allocated for the - * mm. + * Track the highest number of allowed concurrency IDs + * allocated for the mm. */ atomic_t max_nr_cid; /** diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index 21be461ff913..f3b0d1d86622 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -3652,10 +3652,28 @@ static inline int __mm_cid_try_get(struct task_struct *t, struct mm_struct *mm) { struct cpumask *cidmask = mm_cidmask(mm); struct mm_cid __percpu *pcpu_cid = mm->pcpu_cid; - int cid = __this_cpu_read(pcpu_cid->recent_cid); + int cid, max_nr_cid, allowed_max_nr_cid;
+ /* + * After shrinking the number of threads or reducing the number + * of allowed cpus, reduce the value of max_nr_cid so expansion + * of cid allocation will preserve cache locality if the number + * of threads or allowed cpus increase again. + */ + max_nr_cid = atomic_read(&mm->max_nr_cid); + while ((allowed_max_nr_cid = min_t(int, READ_ONCE(mm->nr_cpus_allowed), + atomic_read(&mm->mm_users))), + max_nr_cid > allowed_max_nr_cid) { + /* atomic_try_cmpxchg loads previous mm->max_nr_cid into max_nr_cid. */ + if (atomic_try_cmpxchg(&mm->max_nr_cid, &max_nr_cid, allowed_max_nr_cid)) { + max_nr_cid = allowed_max_nr_cid; + break; + } + } /* Try to re-use recent cid. This improves cache locality. */ - if (!mm_cid_is_unset(cid) && !cpumask_test_and_set_cpu(cid, cidmask)) + cid = __this_cpu_read(pcpu_cid->recent_cid); + if (!mm_cid_is_unset(cid) && cid < max_nr_cid && + !cpumask_test_and_set_cpu(cid, cidmask)) return cid; /* * Expand cid allocation if the maximum number of concurrency @@ -3663,8 +3681,9 @@ static inline int __mm_cid_try_get(struct task_struct *t, struct mm_struct *mm) * and number of threads. Expanding cid allocation as much as * possible improves cache locality. */ - cid = atomic_read(&mm->max_nr_cid); + cid = max_nr_cid; while (cid < READ_ONCE(mm->nr_cpus_allowed) && cid < atomic_read(&mm->mm_users)) { + /* atomic_try_cmpxchg loads previous mm->max_nr_cid into cid. */ if (!atomic_try_cmpxchg(&mm->max_nr_cid, &cid, cid + 1)) continue; if (!cpumask_test_and_set_cpu(cid, cidmask))