| Message ID | 20250210153253.460471-2-gmonaco@redhat.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | [v6,1/3] sched: Compact RSEQ concurrency IDs with reduced threads and affinity | expand |
On 2025-02-10 10:32, Gabriele Monaco wrote: > From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com> > Peter, Ingo, this patch has been ready for inclusion for a while. The rest of this series does not seem to be quite ready yet, but can we at least merge this patch into tip ? Thanks, Mathieu > 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 6b27db7f94963..0234f14f2aa6b 100644 > --- a/include/linux/mm_types.h > +++ b/include/linux/mm_types.h > @@ -875,10 +875,11 @@ 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 38e0e323dda26..606c96b74ebfa 100644 > --- a/kernel/sched/sched.h > +++ b/kernel/sched/sched.h > @@ -3698,10 +3698,28 @@ 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 > @@ -3709,8 +3727,9 @@ 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))
On Thu, Feb 13, 2025 at 09:56:17AM -0500, Mathieu Desnoyers wrote: > On 2025-02-10 10:32, Gabriele Monaco wrote: > > From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com> > > > > Peter, Ingo, this patch has been ready for inclusion for a while. The > rest of this series does not seem to be quite ready yet, but can we at > least merge this patch into tip ? Done; stuck it in queue/sched/urgent for the robots. If that passes I'll push it to tip.
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h index 6b27db7f94963..0234f14f2aa6b 100644 --- a/include/linux/mm_types.h +++ b/include/linux/mm_types.h @@ -875,10 +875,11 @@ 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 38e0e323dda26..606c96b74ebfa 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -3698,10 +3698,28 @@ 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 @@ -3709,8 +3727,9 @@ 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))