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Gleixner"</a></li></ul></li></ul></li></ul></li></ul><div class="threadlist">Patch in this message</div><ul class="threadlist"><li><a href="/lkml/diff/2025/3/8/441/1">Get diff 1</a></li></ul></td><td width="32" rowspan="2" class="c" valign="top"><img src="/images/icornerl.gif" width="32" height="32" alt="/" /></td><td class="c" rowspan="2" valign="top" style="padding-top: 1em"><table><tr><td><table><tr><td class="lp">From</td><td class="rp" itemprop="author">Thomas Gleixner <></td></tr><tr><td class="lp">Subject</td><td class="rp" itemprop="name">[patch V3 14/18] posix-timers: Avoid false cacheline sharing</td></tr><tr><td class="lp">Date</td><td class="rp" itemprop="datePublished">Sat, 8 Mar 2025 17:48:42 +0100 (CET)</td></tr></table></td><td></td></tr></table><pre itemprop="articleBody">struct k_itimer has the hlist_node, which is used for lookup in the hash<br />bucket, and the timer lock in the same cache line.<br /><br />That's obviously bad, if one CPU fiddles with a timer and the other is<br />walking the hash bucket on which that timer is queued.<br /><br />Avoid this by restructuring struct k_itimer, so that the read mostly (only<br />modified during setup and teardown) fields are in the first cache line and<br />the lock and the rest of the fields which get written to are in cacheline<br />2-N.<br /><br />Reduces cacheline contention in a test case of 64 processes creating and<br />accessing 20000 timers each by almost 30% according to perf.<br /><br />Signed-off-by: Thomas Gleixner <tglx@linutronix.de><br /><br />---<br />V2: New patch<br />---<br /> include/linux/posix-timers.h | 21 ++++++++++++---------<br /> kernel/time/posix-timers.c | 4 ++--<br /> 2 files changed, 14 insertions(+), 11 deletions(-)<br /><br />--- a/include/linux/posix-timers.h<br />+++ b/include/linux/posix-timers.h<br />@@ -177,23 +177,26 @@ static inline void posix_cputimers_init_<br /> * @rcu: RCU head for freeing the timer.<br /> */<br /> struct k_itimer {<br />- struct hlist_node list;<br />- struct hlist_node ignored_list;<br />+ /* 1st cacheline contains read-mostly fields */<br /> struct hlist_node t_hash;<br />- spinlock_t it_lock;<br />- const struct k_clock *kclock;<br />- clockid_t it_clock;<br />+ struct hlist_node list;<br /> timer_t it_id;<br />+ clockid_t it_clock;<br />+ int it_sigev_notify;<br />+ enum pid_type it_pid_type;<br />+ struct signal_struct *it_signal;<br />+ const struct k_clock *kclock;<br />+<br />+ /* 2nd cacheline and above contain fields which are modified regularly */<br />+ spinlock_t it_lock;<br /> int it_status;<br /> bool it_sig_periodic;<br /> s64 it_overrun;<br /> s64 it_overrun_last;<br /> unsigned int it_signal_seq;<br /> unsigned int it_sigqueue_seq;<br />- int it_sigev_notify;<br />- enum pid_type it_pid_type;<br /> ktime_t it_interval;<br />- struct signal_struct *it_signal;<br />+ struct hlist_node ignored_list;<br /> union {<br /> struct pid *it_pid;<br /> struct task_struct *it_process;<br />@@ -210,7 +213,7 @@ struct k_itimer {<br /> } alarm;<br /> } it;<br /> struct rcu_head rcu;<br />-};<br />+} ____cacheline_aligned_in_smp;<br /> <br /> void run_posix_cpu_timers(void);<br /> void posix_cpu_timers_exit(struct task_struct *task);<br />--- a/kernel/time/posix-timers.c<br />+++ b/kernel/time/posix-timers.c<br />@@ -260,8 +260,8 @@ static int posix_get_hrtimer_res(clockid<br /> <br /> static __init int init_posix_timers(void)<br /> {<br />- posix_timers_cache = kmem_cache_create("posix_timers_cache", sizeof(struct k_itimer), 0,<br />- SLAB_ACCOUNT, NULL);<br />+ posix_timers_cache = kmem_cache_create("posix_timers_cache", sizeof(struct k_itimer),<br />+ __alignof__(struct k_itimer), SLAB_ACCOUNT, NULL);<br /> return 0;<br /> }<br /> __initcall(init_posix_timers);<br /><br /></pre></td><td width="32" rowspan="2" class="c" valign="top"><img src="/images/icornerr.gif" width="32" height="32" alt="\" /></td></tr><tr><td align="right" valign="bottom"> 聽 </td></tr><tr><td align="right" 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