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Content-Language: en-US To: Sebastian Andrzej Siewior , cgroups@vger.kernel.org, linux-mm@kvack.org Cc: Andrew Morton , Johannes Weiner , Michal Hocko , =?UTF-8?Q?Michal_Koutn=c3=bd?= , Peter Zijlstra , Thomas Gleixner , Vladimir Davydov , Waiman Long References: <20220125164337.2071854-1-bigeasy@linutronix.de> <20220125164337.2071854-3-bigeasy@linutronix.de> From: Vlastimil Babka In-Reply-To: <20220125164337.2071854-3-bigeasy@linutronix.de> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 601A9C000E X-Stat-Signature: j185ur5x8wbkoz5znduxjkzhtrgd6dwd X-Rspam-User: nil Authentication-Results: imf10.hostedemail.com; dkim=pass header.d=suse.cz header.s=susede2_rsa header.b=oYCqCLE2; dkim=pass header.d=suse.cz header.s=susede2_ed25519 header.b=3ic1Mlo3; spf=pass (imf10.hostedemail.com: domain of vbabka@suse.cz designates 195.135.220.28 as permitted sender) smtp.mailfrom=vbabka@suse.cz; dmarc=none X-HE-Tag: 1643191602-729396 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: On 1/25/22 17:43, Sebastian Andrzej Siewior wrote: > The per-CPU counter are modified with the non-atomic modifier. The > consistency is ensured by disabling interrupts for the update. > On non PREEMPT_RT configuration this works because acquiring a > spinlock_t typed lock with the _irq() suffix disables interrupts. On > PREEMPT_RT configurations the RMW operation can be interrupted. > > Another problem is that mem_cgroup_swapout() expects to be invoked with > disabled interrupts because the caller has to acquire a spinlock_t which > is acquired with disabled interrupts. Since spinlock_t never disables > interrupts on PREEMPT_RT the interrupts are never disabled at this > point. > > The code is never called from in_irq() context on PREEMPT_RT therefore > disabling preemption during the update is sufficient on PREEMPT_RT. > The sections which explicitly disable interrupts can remain on > PREEMPT_RT because the sections remain short and they don't involve > sleeping locks (memcg_check_events() is doing nothing on PREEMPT_RT). > > Disable preemption during update of the per-CPU variables which do not > explicitly disable interrupts. > > Signed-off-by: Sebastian Andrzej Siewior So it's like c68ed7945701 ("mm/vmstat: protect per cpu variables with preempt disable on RT") but we still don't want a wrapper of those constructs so they don't spread further, right :) Acked-by: Vlastimil Babka > --- > mm/memcontrol.c | 21 +++++++++++++++++++-- > 1 file changed, 19 insertions(+), 2 deletions(-) > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index 36d27db673ca9..3d1b7cdd83db0 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -667,6 +667,8 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx, > pn = container_of(lruvec, struct mem_cgroup_per_node, lruvec); > memcg = pn->memcg; > > + if (IS_ENABLED(CONFIG_PREEMPT_RT)) > + preempt_disable(); > /* Update memcg */ > __this_cpu_add(memcg->vmstats_percpu->state[idx], val); > > @@ -674,6 +676,8 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx, > __this_cpu_add(pn->lruvec_stats_percpu->state[idx], val); > > memcg_rstat_updated(memcg, val); > + if (IS_ENABLED(CONFIG_PREEMPT_RT)) > + preempt_enable(); > } > > /** > @@ -756,8 +760,12 @@ void __count_memcg_events(struct mem_cgroup *memcg, enum vm_event_item idx, > if (mem_cgroup_disabled()) > return; > > + if (IS_ENABLED(PREEMPT_RT)) > + preempt_disable(); > __this_cpu_add(memcg->vmstats_percpu->events[idx], count); > memcg_rstat_updated(memcg, count); > + if (IS_ENABLED(PREEMPT_RT)) > + preempt_enable(); > } > > static unsigned long memcg_events(struct mem_cgroup *memcg, int event) > @@ -7194,9 +7202,18 @@ void mem_cgroup_swapout(struct page *page, swp_entry_t entry) > * i_pages lock which is taken with interrupts-off. It is > * important here to have the interrupts disabled because it is the > * only synchronisation we have for updating the per-CPU variables. > + * On PREEMPT_RT interrupts are never disabled and the updates to per-CPU > + * variables are synchronised by keeping preemption disabled. > */ > - VM_BUG_ON(!irqs_disabled()); > - mem_cgroup_charge_statistics(memcg, -nr_entries); > + if (!IS_ENABLED(CONFIG_PREEMPT_RT)) { > + VM_BUG_ON(!irqs_disabled()); > + mem_cgroup_charge_statistics(memcg, -nr_entries); > + } else { > + preempt_disable(); > + mem_cgroup_charge_statistics(memcg, -nr_entries); > + preempt_enable(); > + } > + > memcg_check_events(memcg, page_to_nid(page)); > > css_put(&memcg->css);