linux/kernel
Linus Torvalds fda31c5029 signal: avoid double atomic counter increments for user accounting
When queueing a signal, we increment both the users count of pending
signals (for RLIMIT_SIGPENDING tracking) and we increment the refcount
of the user struct itself (because we keep a reference to the user in
the signal structure in order to correctly account for it when freeing).

That turns out to be fairly expensive, because both of them are atomic
updates, and particularly under extreme signal handling pressure on big
machines, you can get a lot of cache contention on the user struct.
That can then cause horrid cacheline ping-pong when you do these
multiple accesses.

So change the reference counting to only pin the user for the _first_
pending signal, and to unpin it when the last pending signal is
dequeued.  That means that when a user sees a lot of concurrent signal
queuing - which is the only situation when this matters - the only
atomic access needed is generally the 'sigpending' count update.

This was noticed because of a particularly odd timing artifact on a
dual-socket 96C/192T Cascade Lake platform: when you get into bad
contention, on that machine for some reason seems to be much worse when
the contention happens in the upper 32-byte half of the cacheline.

As a result, the kernel test robot will-it-scale 'signal1' benchmark had
an odd performance regression simply due to random alignment of the
'struct user_struct' (and pointed to a completely unrelated and
apparently nonsensical commit for the regression).

Avoiding the double increments (and decrements on the dequeueing side,
of course) makes for much less contention and hugely improved
performance on that will-it-scale microbenchmark.

Quoting Feng Tang:

 "It makes a big difference, that the performance score is tripled! bump
  from original 17000 to 54000. Also the gap between 5.0-rc6 and
  5.0-rc6+Jiri's patch is reduced to around 2%"

[ The "2% gap" is the odd cacheline placement difference on that
  platform: under the extreme contention case, the effect of which half
  of the cacheline was hot was 5%, so with the reduced contention the
  odd timing artifact is reduced too ]

It does help in the non-contended case too, but is not nearly as
noticeable.

Reported-and-tested-by: Feng Tang <feng.tang@intel.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Huang, Ying <ying.huang@intel.com>
Cc: Philip Li <philip.li@intel.com>
Cc: Andi Kleen <andi.kleen@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2020-02-26 09:54:03 -08:00
..
bpf bpf: Fix a potential deadlock with bpf_map_do_batch 2020-02-19 16:01:25 -08:00
cgroup Merge branch 'for-5.6-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup 2020-02-10 17:07:05 -08:00
configs
debug Revert "kdb: Get rid of confusing diag msg from "rd" if current task has no regs" 2020-02-06 11:40:09 +00:00
dma dma-direct: improve DMA mask overflow reporting 2020-02-05 18:53:41 +01:00
events A set of fixes and improvements for the perf subsystem: 2020-02-09 12:04:09 -08:00
gcov Revert "um: Enable CONFIG_CONSTRUCTORS" 2020-01-19 22:42:06 +01:00
irq genirq/proc: Reject invalid affinity masks (again) 2020-02-14 09:43:17 +01:00
livepatch New tracing features: 2019-11-27 11:42:01 -08:00
locking proc: convert everything to "struct proc_ops" 2020-02-04 03:05:26 +00:00
power ACPI: PM: s2idle: Avoid possible race related to the EC GPE 2020-02-11 10:11:02 +01:00
printk printk: fix exclusive_console replaying 2020-01-02 16:15:04 +01:00
rcu rcu: Forgive slow expedited grace periods at boot time 2020-01-25 12:00:40 -08:00
sched Merge branch 'sched-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2020-02-15 12:51:22 -08:00
time y2038: remove unused time32 interfaces 2020-02-21 11:22:15 -08:00
trace Various fixes: 2020-02-11 16:39:18 -08:00
.gitignore
acct.c acct: stop using get_seconds() 2019-12-18 18:07:31 +01:00
async.c treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 441 2019-06-05 17:37:17 +02:00
audit_fsnotify.c treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 157 2019-05-30 11:26:37 -07:00
audit_tree.c
audit_watch.c audit_get_nd(): don't unlock parent too early 2019-11-10 11:56:55 -05:00
audit.c audit: Add __rcu annotation to RCU pointer 2019-12-09 15:19:03 -05:00
audit.h audit/stable-5.3 PR 20190702 2019-07-08 18:55:42 -07:00
auditfilter.c audit/stable-5.3 PR 20190702 2019-07-08 18:55:42 -07:00
auditsc.c Revert "bpf: Emit audit messages upon successful prog load and unload" 2019-11-23 09:56:02 -08:00
backtracetest.c treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 441 2019-06-05 17:37:17 +02:00
bounds.c
capability.c
compat.c y2038: remove unused time32 interfaces 2020-02-21 11:22:15 -08:00
configs.c proc: convert everything to "struct proc_ops" 2020-02-04 03:05:26 +00:00
context_tracking.c context_tracking: Rename context_tracking_is_enabled() => context_tracking_enabled() 2019-10-29 10:01:12 +01:00
cpu_pm.c treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 282 2019-06-05 17:36:37 +02:00
cpu.c Merge branch 'sched-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2020-01-28 10:07:09 -08:00
crash_core.c treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 230 2019-06-19 17:09:06 +02:00
crash_dump.c
cred.c Merge branch 'dhowells' (patches from DavidH) 2020-01-14 09:56:31 -08:00
delayacct.c
dma.c
elfcore.c kernel/elfcore.c: include proper prototypes 2019-09-25 17:51:39 -07:00
exec_domain.c
exit.c for-linus-2020-01-03 2020-01-03 11:17:14 -08:00
extable.c bpf: Allow to resolve bpf trampoline and dispatcher in unwind 2020-01-25 07:12:40 -08:00
fail_function.c fail_function: no need to check return value of debugfs_create functions 2019-06-03 15:49:06 +02:00
fork.c hmm related patches for 5.6 2020-01-29 19:56:50 -08:00
freezer.c Revert "libata, freezer: avoid block device removal while system is frozen" 2019-10-06 09:11:37 -06:00
futex.c futex: Fix kernel-doc notation warning 2020-01-09 13:23:40 +01:00
gen_kheaders.sh kheaders: explain why include/config/autoconf.h is excluded from md5sum 2019-11-11 20:10:01 +09:00
groups.c
hung_task.c
iomem.c mm/nvdimm: add is_ioremap_addr and use that to check ioremap address 2019-07-12 11:05:40 -07:00
irq_work.c irq_work: Fix IRQ_WORK_BUSY bit clearing 2019-11-15 10:48:37 +01:00
jump_label.c jump_label: Don't warn on __exit jump entries 2019-08-29 15:10:10 +01:00
kallsyms.c Kbuild updates for v5.6 (2nd) 2020-02-09 16:05:50 -08:00
kcmp.c
Kconfig.freezer
Kconfig.hz
Kconfig.locks sched/rt, locking: Use CONFIG_PREEMPTION 2019-12-08 14:37:36 +01:00
Kconfig.preempt sched/Kconfig: Fix spelling mistake in user-visible help text 2019-11-12 11:35:32 +01:00
kcov.c kcov: remote coverage support 2019-12-04 19:44:14 -08:00
kexec_core.c kexec: add machine_kexec_post_load() 2020-01-08 16:32:55 +00:00
kexec_elf.c kexec_elf: support 32 bit ELF files 2019-09-06 23:58:44 +02:00
kexec_file.c kexec: add machine_kexec_post_load() 2020-01-08 16:32:55 +00:00
kexec_internal.h kexec: add machine_kexec_post_load() 2020-01-08 16:32:55 +00:00
kexec.c kexec: add machine_kexec_post_load() 2020-01-08 16:32:55 +00:00
kheaders.c
kmod.c
kprobes.c kprobes: Fix optimize_kprobe()/unoptimize_kprobe() cancellation logic 2020-01-09 12:40:13 +01:00
ksysfs.c treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 170 2019-05-30 11:26:39 -07:00
kthread.c kthread: make __kthread_queue_delayed_work static 2019-10-16 09:20:58 -07:00
latencytop.c proc: convert everything to "struct proc_ops" 2020-02-04 03:05:26 +00:00
Makefile kcov: ignore fault-inject and stacktrace 2020-01-31 10:30:41 -08:00
module_signature.c MODSIGN: Export module signature definitions 2019-08-05 18:39:56 -04:00
module_signing.c MODSIGN: Export module signature definitions 2019-08-05 18:39:56 -04:00
module-internal.h
module.c proc: convert everything to "struct proc_ops" 2020-02-04 03:05:26 +00:00
notifier.c kernel/notifier.c: remove blocking_notifier_chain_cond_register() 2019-12-04 19:44:12 -08:00
nsproxy.c ns: Introduce Time Namespace 2020-01-14 12:20:48 +01:00
padata.c padata: update documentation 2019-12-11 16:37:02 +08:00
panic.c locking/refcount: Remove unused 'refcount_error_report()' function 2019-11-25 09:15:42 +01:00
params.c lockdown: Lock down module params that specify hardware parameters (eg. ioport) 2019-08-19 21:54:16 -07:00
pid_namespace.c fork: extend clone3() to support setting a PID 2019-11-15 23:49:22 +01:00
pid.c pid: Implement pidfd_getfd syscall 2020-01-13 21:49:36 +01:00
profile.c proc: convert everything to "struct proc_ops" 2020-02-04 03:05:26 +00:00
ptrace.c ptrace: reintroduce usage of subjective credentials in ptrace_has_cap() 2020-01-18 13:51:39 +01:00
range.c
reboot.c
relay.c
resource.c mm/memory_hotplug.c: use PFN_UP / PFN_DOWN in walk_system_ram_range() 2019-09-24 15:54:09 -07:00
rseq.c rseq: Reject unknown flags on rseq unregister 2019-12-25 10:41:20 +01:00
seccomp.c seccomp: Check that seccomp_notif is zeroed out by the user 2020-01-02 13:03:45 -08:00
signal.c signal: avoid double atomic counter increments for user accounting 2020-02-26 09:54:03 -08:00
smp.c smp: Remove superfluous cond_func check in smp_call_function_many_cond() 2020-01-28 15:43:00 +01:00
smpboot.c
smpboot.h
softirq.c Merge branch 'irq-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2019-07-08 11:01:13 -07:00
stackleak.c
stacktrace.c stacktrace: Get rid of unneeded '!!' pattern 2019-11-11 10:30:59 +01:00
stop_machine.c stop_machine: Make stop_cpus() static 2020-01-17 10:19:21 +01:00
sys_ni.c y2038: allow disabling time32 system calls 2019-11-15 14:38:30 +01:00
sys.c prctl: PR_{G,S}ET_IO_FLUSHER to support controlling memory reclaim 2020-01-28 10:09:51 +01:00
sysctl_binary.c sysctl: Remove the sysctl system call 2019-11-26 13:03:56 -06:00
sysctl-test.c kunit: allow kunit tests to be loaded as a module 2020-01-09 16:42:29 -07:00
sysctl.c s390: remove obsolete ieee_emulation_warnings 2020-02-19 13:51:46 +01:00
task_work.c
taskstats.c taskstats: fix data-race 2019-12-04 15:18:39 +01:00
test_kprobes.c
torture.c torture: Remove exporting of internal functions 2019-08-01 14:30:22 -07:00
tracepoint.c The main changes in this release include: 2019-07-18 11:51:00 -07:00
tsacct.c tsacct: add 64-bit btime field 2019-12-18 18:07:31 +01:00
ucount.c proc/sysctl: add shared variables for range check 2019-07-18 17:08:07 -07:00
uid16.c
uid16.h
umh.c
up.c smp/up: Make smp_call_function_single() match SMP semantics 2020-02-07 15:34:12 +01:00
user_namespace.c Keyrings namespacing 2019-07-08 19:36:47 -07:00
user-return-notifier.c
user.c Keyrings namespacing 2019-07-08 19:36:47 -07:00
utsname_sysctl.c treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 441 2019-06-05 17:37:17 +02:00
utsname.c treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 441 2019-06-05 17:37:17 +02:00
watchdog_hld.c
watchdog.c watchdog/softlockup: Enforce that timestamp is valid on boot 2020-01-17 11:19:22 +01:00
workqueue_internal.h
workqueue.c Merge branch 'sched-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2020-01-28 10:07:09 -08:00