On Thu, Jul 11, 2024 at 6:28 AM Jesper Dangaard Brouer <hawk@xxxxxxxxxx> wrote: > > Avoid lock contention on the global cgroup rstat lock caused by kswapd > starting on all NUMA nodes simultaneously. At Cloudflare, we observed > massive issues due to kswapd and the specific mem_cgroup_flush_stats() > call inlined in shrink_node, which takes the rstat lock. > > On our 12 NUMA node machines, each with a kswapd kthread per NUMA node, > we noted severe lock contention on the rstat lock. This contention > causes 12 CPUs to waste cycles spinning every time kswapd runs. > Fleet-wide stats (/proc/N/schedstat) for kthreads revealed that we are > burning an average of 20,000 CPU cores fleet-wide on kswapd, primarily > due to spinning on the rstat lock. > > Help reviewers follow code: __alloc_pages_slowpath calls wake_all_kswapds > causing all kswapdN threads to wake up simultaneously. The kswapd thread > invokes shrink_node (via balance_pgdat) triggering the cgroup rstat flush > operation as part of its work. This results in kernel self-induced rstat > lock contention by waking up all kswapd threads simultaneously. Leveraging > this detail: balance_pgdat() have NULL value in target_mem_cgroup, this > cause mem_cgroup_flush_stats() to do flush with root_mem_cgroup. > > To avoid this kind of thundering herd problem, kernel previously had a > "stats_flush_ongoing" concept, but this was removed as part of commit > 7d7ef0a4686a ("mm: memcg: restore subtree stats flushing"). This patch > reintroduce and generalized the concept to apply to all users of cgroup > rstat, not just memcg. > > If there is an ongoing rstat flush, and current cgroup is a descendant, > then it is unnecessary to do the flush. For callers to still see updated > stats, wait for ongoing flusher to complete before returning, but add > timeout as stats are already inaccurate given updaters keeps running. > > Fixes: 7d7ef0a4686a ("mm: memcg: restore subtree stats flushing"). > Signed-off-by: Jesper Dangaard Brouer <hawk@xxxxxxxxxx> Thanks for working on this, Jesper! I love the data you collected here! I think the commit subject and message should be changed to better describe the patch. This is a patch that exclusively modifies cgroup code, yet the subject is about kswapd. This change affects all users of rstat flushing. I think a better subject would be: "cgroup/rstat: avoid flushing if there is an ongoing overlapping flush" or similar. The commit message should first describe the cgroup change, and then use kswapd as a brief example/illustration of how the problem manifests in practice. You should also include a brief summary of the numbers you collected from prod. > --- > V6: https://lore.kernel.org/all/172052399087.2357901.4955042377343593447.stgit@firesoul/ > V5: https://lore.kernel.org/all/171956951930.1897969.8709279863947931285.stgit@firesoul/ > V4: https://lore.kernel.org/all/171952312320.1810550.13209360603489797077.stgit@firesoul/ > V3: https://lore.kernel.org/all/171943668946.1638606.1320095353103578332.stgit@firesoul/ > V2: https://lore.kernel.org/all/171923011608.1500238.3591002573732683639.stgit@firesoul/ > V1: https://lore.kernel.org/all/171898037079.1222367.13467317484793748519.stgit@firesoul/ > RFC: https://lore.kernel.org/all/171895533185.1084853.3033751561302228252.stgit@firesoul/ > > include/linux/cgroup-defs.h | 2 + > kernel/cgroup/rstat.c | 95 ++++++++++++++++++++++++++++++++++++++----- > 2 files changed, 85 insertions(+), 12 deletions(-) > > diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h > index b36690ca0d3f..a33b37514c29 100644 > --- a/include/linux/cgroup-defs.h > +++ b/include/linux/cgroup-defs.h > @@ -548,6 +548,8 @@ struct cgroup { > #ifdef CONFIG_BPF_SYSCALL > struct bpf_local_storage __rcu *bpf_cgrp_storage; > #endif > + /* completion queue for cgrp_rstat_ongoing_flusher */ > + struct completion flush_done; > > /* All ancestors including self */ > struct cgroup *ancestors[]; > diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c > index fb8b49437573..fe2a81a310bb 100644 > --- a/kernel/cgroup/rstat.c > +++ b/kernel/cgroup/rstat.c > @@ -2,6 +2,7 @@ > #include "cgroup-internal.h" > > #include <linux/sched/cputime.h> > +#include <linux/completion.h> > > #include <linux/bpf.h> > #include <linux/btf.h> > @@ -11,6 +12,7 @@ > > static DEFINE_SPINLOCK(cgroup_rstat_lock); > static DEFINE_PER_CPU(raw_spinlock_t, cgroup_rstat_cpu_lock); > +static struct cgroup *cgrp_rstat_ongoing_flusher = NULL; > > static void cgroup_base_stat_flush(struct cgroup *cgrp, int cpu); > > @@ -279,17 +281,32 @@ __bpf_hook_end(); > * value -1 is used when obtaining the main lock else this is the CPU > * number processed last. > */ > -static inline void __cgroup_rstat_lock(struct cgroup *cgrp, int cpu_in_loop) > +static inline bool __cgroup_rstat_trylock(struct cgroup *cgrp, int cpu_in_loop) > +{ > + bool locked; > + > + locked = spin_trylock_irq(&cgroup_rstat_lock); > + if (!locked) > + trace_cgroup_rstat_lock_contended(cgrp, cpu_in_loop, true); > + else > + trace_cgroup_rstat_locked(cgrp, cpu_in_loop, false); > + > + return locked; > +} > + > +static inline void __cgroup_rstat_lock(struct cgroup *cgrp, int cpu_in_loop, > + bool check_contention) > __acquires(&cgroup_rstat_lock) > { > - bool contended; > + bool locked = false; > > - contended = !spin_trylock_irq(&cgroup_rstat_lock); > - if (contended) { > - trace_cgroup_rstat_lock_contended(cgrp, cpu_in_loop, contended); > + if (check_contention) > + locked = __cgroup_rstat_trylock(cgrp, cpu_in_loop); > + > + if (!locked) { > spin_lock_irq(&cgroup_rstat_lock); > + trace_cgroup_rstat_locked(cgrp, cpu_in_loop, check_contention); > } > - trace_cgroup_rstat_locked(cgrp, cpu_in_loop, contended); > } I will let Tejun and others weigh in about what tracepoints we should have going forward and how they should be structured (which also includes patch 2). I see the tremendous value they have to collect data from prod, but I do not see similar existing tracepoints, and I am not sure if the branching here could have an effect when the tracepoints are off. > > static inline void __cgroup_rstat_unlock(struct cgroup *cgrp, int cpu_in_loop) > @@ -299,6 +316,53 @@ static inline void __cgroup_rstat_unlock(struct cgroup *cgrp, int cpu_in_loop) > spin_unlock_irq(&cgroup_rstat_lock); > } > > +#define MAX_WAIT msecs_to_jiffies(100) > +/* Trylock helper that also checks for on ongoing flusher */ > +static bool cgroup_rstat_trylock_flusher(struct cgroup *cgrp) > +{ > + struct cgroup *ongoing; > + bool locked; > + > + /* Check if ongoing flusher is already taking care of this, if nit: I think commonly the comment would start on a new line after /*. > + * we are a descendant skip work, but wait for ongoing flusher > + * to complete work. > + */ > +retry: > + ongoing = READ_ONCE(cgrp_rstat_ongoing_flusher); > + if (ongoing && cgroup_is_descendant(cgrp, ongoing)) { > + wait_for_completion_interruptible_timeout( > + &ongoing->flush_done, MAX_WAIT); > + /* TODO: Add tracepoint here */ > + return false; > + } > + > + locked = __cgroup_rstat_trylock(cgrp, -1); > + if (!locked) { > + /* Contended: Handle loosing race for ongoing flusher */ nit: losing > + if (!ongoing && READ_ONCE(cgrp_rstat_ongoing_flusher)) > + goto retry; > + > + __cgroup_rstat_lock(cgrp, -1, false); > + } > + /* Obtained lock, record this cgrp as the ongoing flusher */ Do we want a comment here to explain why there could be an existing ongoing flusher (i.e. due to multiple ongoing flushers)? I think it's not super obvious. > + ongoing = READ_ONCE(cgrp_rstat_ongoing_flusher); > + if (!ongoing) { > + reinit_completion(&cgrp->flush_done); > + WRITE_ONCE(cgrp_rstat_ongoing_flusher, cgrp); > + } > + return true; /* locked */ Would it be better to explain the return value of the function in the comment above it? > +} > + > +static void cgroup_rstat_unlock_flusher(struct cgroup *cgrp) > +{ > + /* Detect if we are the ongoing flusher */ I think this is a bit obvious. > + if (cgrp == READ_ONCE(cgrp_rstat_ongoing_flusher)) { > + WRITE_ONCE(cgrp_rstat_ongoing_flusher, NULL); > + complete_all(&cgrp->flush_done); > + } > + __cgroup_rstat_unlock(cgrp, -1); > +} > + > /* see cgroup_rstat_flush() */ > static void cgroup_rstat_flush_locked(struct cgroup *cgrp) > __releases(&cgroup_rstat_lock) __acquires(&cgroup_rstat_lock) > @@ -328,7 +392,7 @@ static void cgroup_rstat_flush_locked(struct cgroup *cgrp) > __cgroup_rstat_unlock(cgrp, cpu); > if (!cond_resched()) > cpu_relax(); > - __cgroup_rstat_lock(cgrp, cpu); > + __cgroup_rstat_lock(cgrp, cpu, true); > } > } > } > @@ -350,9 +414,11 @@ __bpf_kfunc void cgroup_rstat_flush(struct cgroup *cgrp) > { > might_sleep(); > > - __cgroup_rstat_lock(cgrp, -1); > + if (!cgroup_rstat_trylock_flusher(cgrp)) > + return; > + > cgroup_rstat_flush_locked(cgrp); > - __cgroup_rstat_unlock(cgrp, -1); > + cgroup_rstat_unlock_flusher(cgrp); > } > > /** > @@ -368,8 +434,11 @@ void cgroup_rstat_flush_hold(struct cgroup *cgrp) > __acquires(&cgroup_rstat_lock) > { > might_sleep(); > - __cgroup_rstat_lock(cgrp, -1); > - cgroup_rstat_flush_locked(cgrp); > + > + if (cgroup_rstat_trylock_flusher(cgrp)) > + cgroup_rstat_flush_locked(cgrp); > + else > + __cgroup_rstat_lock(cgrp, -1, false); > } > > /** > @@ -379,7 +448,7 @@ void cgroup_rstat_flush_hold(struct cgroup *cgrp) > void cgroup_rstat_flush_release(struct cgroup *cgrp) > __releases(&cgroup_rstat_lock) > { > - __cgroup_rstat_unlock(cgrp, -1); > + cgroup_rstat_unlock_flusher(cgrp); > } > > int cgroup_rstat_init(struct cgroup *cgrp) > @@ -401,6 +470,8 @@ int cgroup_rstat_init(struct cgroup *cgrp) > u64_stats_init(&rstatc->bsync); > } > > + init_completion(&cgrp->flush_done); > + > return 0; > } > > >