# git rev-parse -q --verify fe67f4dd8daa252eb9aa7acb61555f3cc3c1ce4c^{commit} fe67f4dd8daa252eb9aa7acb61555f3cc3c1ce4c already have revision, skipping fetch # git checkout -q -f -B kisskb fe67f4dd8daa252eb9aa7acb61555f3cc3c1ce4c # git clean -qxdf # < git log -1 # commit fe67f4dd8daa252eb9aa7acb61555f3cc3c1ce4c # Author: Linus Torvalds # Date: Tue Aug 24 10:39:25 2021 -0700 # # pipe: do FASYNC notifications for every pipe IO, not just state changes # # It turns out that the SIGIO/FASYNC situation is almost exactly the same # as the EPOLLET case was: user space really wants to be notified after # every operation. # # Now, in a perfect world it should be sufficient to only notify user # space on "state transitions" when the IO state changes (ie when a pipe # goes from unreadable to readable, or from unwritable to writable). User # space should then do as much as possible - fully emptying the buffer or # what not - and we'll notify it again the next time the state changes. # # But as with EPOLLET, we have at least one case (stress-ng) where the # kernel sent SIGIO due to the pipe being marked for asynchronous # notification, but the user space signal handler then didn't actually # necessarily read it all before returning (it read more than what was # written, but since there could be multiple writes, it could leave data # pending). # # The user space code then expected to get another SIGIO for subsequent # writes - even though the pipe had been readable the whole time - and # would only then read more. # # This is arguably a user space bug - and Colin King already fixed the # stress-ng code in question - but the kernel regression rules are clear: # it doesn't matter if kernel people think that user space did something # silly and wrong. What matters is that it used to work. # # So if user space depends on specific historical kernel behavior, it's a # regression when that behavior changes. It's on us: we were silly to # have that non-optimal historical behavior, and our old kernel behavior # was what user space was tested against. # # Because of how the FASYNC notification was tied to wakeup behavior, this # was first broken by commits f467a6a66419 and 1b6b26ae7053 ("pipe: fix # and clarify pipe read/write wakeup logic"), but at the time it seems # nobody noticed. Probably because the stress-ng problem case ends up # being timing-dependent too. # # It was then unwittingly fixed by commit 3a34b13a88ca ("pipe: make pipe # writes always wake up readers") only to be broken again when by commit # 3b844826b6c6 ("pipe: avoid unnecessary EPOLLET wakeups under normal # loads"). # # And at that point the kernel test robot noticed the performance # refression in the stress-ng.sigio.ops_per_sec case. So the "Fixes" tag # below is somewhat ad hoc, but it matches when the issue was noticed. # # Fix it for good (knock wood) by simply making the kill_fasync() case # separate from the wakeup case. FASYNC is quite rare, and we clearly # shouldn't even try to use the "avoid unnecessary wakeups" logic for it. # # Link: https://lore.kernel.org/lkml/20210824151337.GC27667@xsang-OptiPlex-9020/ # Fixes: 3b844826b6c6 ("pipe: avoid unnecessary EPOLLET wakeups under normal loads") # Reported-by: kernel test robot # Tested-by: Oliver Sang # Cc: Eric Biederman # Cc: Colin Ian King # Signed-off-by: Linus Torvalds # < /opt/cross/kisskb/br-sparc64-full-2016.08-613-ge98b4dd/bin/sparc64-linux-gcc --version # < /opt/cross/kisskb/br-sparc64-full-2016.08-613-ge98b4dd/bin/sparc64-linux-ld --version # < git log --format=%s --max-count=1 fe67f4dd8daa252eb9aa7acb61555f3cc3c1ce4c # < make -s -j 32 ARCH=sparc O=/kisskb/build/linus_sparc-allmodconfig_sparc64 CROSS_COMPILE=/opt/cross/kisskb/br-sparc64-full-2016.08-613-ge98b4dd/bin/sparc64-linux- allmodconfig # Added to kconfig CONFIG_64BIT=n # Added to kconfig CONFIG_BUILD_DOCSRC=n # Added to kconfig CONFIG_HAVE_FTRACE_MCOUNT_RECORD=n # Added to kconfig CONFIG_SAMPLES=n # Added to kconfig CONFIG_MODULE_SIG=n # < make -s -j 32 ARCH=sparc O=/kisskb/build/linus_sparc-allmodconfig_sparc64 CROSS_COMPILE=/opt/cross/kisskb/br-sparc64-full-2016.08-613-ge98b4dd/bin/sparc64-linux- help # make -s -j 32 ARCH=sparc O=/kisskb/build/linus_sparc-allmodconfig_sparc64 CROSS_COMPILE=/opt/cross/kisskb/br-sparc64-full-2016.08-613-ge98b4dd/bin/sparc64-linux- olddefconfig # make -s -j 32 ARCH=sparc O=/kisskb/build/linus_sparc-allmodconfig_sparc64 CROSS_COMPILE=/opt/cross/kisskb/br-sparc64-full-2016.08-613-ge98b4dd/bin/sparc64-linux- :1515:2: warning: #warning syscall clone3 not implemented [-Wcpp] /kisskb/src/drivers/tty/serial/sunzilog.c:1128:13: warning: 'sunzilog_putchar' defined but not used [-Wunused-function] static void sunzilog_putchar(struct uart_port *port, int ch) ^ /kisskb/src/drivers/input/joystick/analog.c:160:2: warning: #warning Precise timer not defined for this architecture. [-Wcpp] #warning Precise timer not defined for this architecture. ^ WARNING: modpost: EXPORT symbol "bzero_1page" [vmlinux] version ... Is "bzero_1page" prototyped in ? WARNING: modpost: EXPORT symbol "___rw_read_exit" [vmlinux] version ... Is "___rw_read_exit" prototyped in ? WARNING: modpost: EXPORT symbol "___rw_read_try" [vmlinux] version ... Is "___rw_read_try" prototyped in ? WARNING: modpost: EXPORT symbol "__ndelay" [vmlinux] version ... Is "__ndelay" prototyped in ? WARNING: modpost: EXPORT symbol "__ashrdi3" [vmlinux] version ... Is "__ashrdi3" prototyped in ? WARNING: modpost: EXPORT symbol "empty_zero_page" [vmlinux] version ... Is "empty_zero_page" prototyped in ? WARNING: modpost: EXPORT symbol "__udelay" [vmlinux] version ... Is "__udelay" prototyped in ? WARNING: modpost: EXPORT symbol "__lshrdi3" [vmlinux] version ... Is "__lshrdi3" prototyped in ? WARNING: modpost: EXPORT symbol "___rw_write_enter" [vmlinux] version ... Is "___rw_write_enter" prototyped in ? WARNING: modpost: EXPORT symbol "__ashldi3" [vmlinux] version ... Is "__ashldi3" prototyped in ? WARNING: modpost: EXPORT symbol "__copy_1page" [vmlinux] version ... Is "__copy_1page" prototyped in ? WARNING: modpost: EXPORT symbol "__muldi3" [vmlinux] version ... Is "__muldi3" prototyped in ? WARNING: modpost: EXPORT symbol "__divdi3" [vmlinux] version ... Is "__divdi3" prototyped in ? WARNING: modpost: EXPORT symbol "___rw_read_enter" [vmlinux] version ... Is "___rw_read_enter" prototyped in ? arch/sparc/kernel/head_32.o: In function `current_pc': arch/sparc/kernel/head_32.o:(.head.text+0x5040): relocation truncated to fit: R_SPARC_WDISP22 against `.init.text' arch/sparc/kernel/head_32.o: In function `halt_notsup': arch/sparc/kernel/head_32.o:(.head.text+0x5100): relocation truncated to fit: R_SPARC_WDISP22 against `.init.text' arch/sparc/kernel/head_32.o: In function `leon_init': arch/sparc/kernel/head_32.o:(.init.text+0xa4): relocation truncated to fit: R_SPARC_WDISP22 against symbol `leon_smp_cpu_startup' defined in .text section in arch/sparc/kernel/trampoline_32.o arch/sparc/kernel/process_32.o:(.fixup+0x4): relocation truncated to fit: R_SPARC_WDISP22 against `.text' arch/sparc/kernel/process_32.o:(.fixup+0xc): relocation truncated to fit: R_SPARC_WDISP22 against `.text' arch/sparc/kernel/signal_32.o:(.fixup+0x4): relocation truncated to fit: R_SPARC_WDISP22 against `.text' arch/sparc/kernel/signal_32.o:(.fixup+0x10): relocation truncated to fit: R_SPARC_WDISP22 against `.text' arch/sparc/kernel/signal_32.o:(.fixup+0x1c): relocation truncated to fit: R_SPARC_WDISP22 against `.text' arch/sparc/kernel/signal_32.o:(.fixup+0x28): relocation truncated to fit: R_SPARC_WDISP22 against `.text' arch/sparc/kernel/signal_32.o:(.fixup+0x34): relocation truncated to fit: R_SPARC_WDISP22 against `.text' arch/sparc/kernel/signal_32.o:(.fixup+0x40): additional relocation overflows omitted from the output make[1]: *** [/kisskb/src/Makefile:1176: vmlinux] Error 1 make: *** [Makefile:220: __sub-make] Error 2 Command 'make -s -j 32 ARCH=sparc O=/kisskb/build/linus_sparc-allmodconfig_sparc64 CROSS_COMPILE=/opt/cross/kisskb/br-sparc64-full-2016.08-613-ge98b4dd/bin/sparc64-linux- ' returned non-zero exit status 2 # rm -rf /kisskb/build/linus_sparc-allmodconfig_sparc64 # Build took: 0:07:47.193147