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McKenney" , Shuah Khan Subject: [PATCH v6 1/3] sched: Compact RSEQ concurrency IDs with reduced threads and affinity Date: Mon, 10 Feb 2025 16:32:50 +0100 Message-ID: <20250210153253.460471-2-gmonaco@redhat.com> In-Reply-To: <20250210153253.460471-1-gmonaco@redhat.com> References: <20250210153253.460471-1-gmonaco@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.111 X-Rspam-User: X-Rspamd-Queue-Id: 438D3180012 X-Rspamd-Server: rspam12 X-Stat-Signature: qanwa75fjwd5cq3qxbz5dyos5t5r3hso X-HE-Tag: 1739201601-725384 X-HE-Meta: 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 w+oXoRsy eHttuBOg25SG7GpayixPwGu4qAWF9vgq1GJZFlKA9hHw7MHK1njppkEzVYJZpqs2Ll8RHUojgDFn84rIPeg472qYMNPFsUWeoF7otwwzWVMmgfCpK2BscbJ1TaHgigdYTdHURgNzj2X+b9b6aLL6zhouDQqNZAA6wxp7i3GH6LEjclMlGRK/h6cRDwJ8bd3h+wNtaNdsTlZyYmeNo8MTGbV6qhkgZGqkscxOrJ3aE3y2qIyHJMbdmwa4MNGT41Bc3NwgbuW2JUptjs2wLjmMyn/jKprbrXWAXp4sPHz1d/xLS40ragqOCnmRLU7vpxOyoeLaF3H4oUf9DW/lwP0jdf9fNcSIlNEA+9CYEhGrid1V7YhhIHoDv1JgytpdBr85PQnSAgdhg66OXbygA95eW5Ki05Fk1mLknswP90RVDQX7UsmwIYk+lrzZstUi3uCWRCtfZmYl25N9vs21Q22tJGEC7nU8WJ8klV1C67D3446NSiz4= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: From: Mathieu Desnoyers 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) Cc: Marco Elver Cc: Ingo Molnar Tested-by: Gabriele Monaco Signed-off-by: Mathieu Desnoyers Signed-off-by: Gabriele Monaco --- 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))