* Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> wrote: > > I've not been following the background to this, > > We need/want to do a user-space stack walk from IRQ/NMI context. > The NMI bit means we cannot simply use __copy_from_user_inatomic() > since that will still fault (albeit not page), and the fault > return path invokes IRET which will terminate the NMI context. The goal is to allow 'perf' (see tools/perf/) non-flat categorizations like the sample output in the (pending) commit (see it attached further below). Here's the kind of output it allows: $ perf record -g -m 512 -f -- make -j32 kernel $ perf report -s s --syscalls | grep '\[k\]' | grep -v nmi 4.14% [k] do_page_fault 1.20% [k] sys_write 1.10% [k] sys_open 0.63% [k] sys_exit_group 0.48% [k] smp_apic_timer_interrupt 0.37% [k] sys_read 0.37% [k] sys_execve 0.20% [k] sys_mmap 0.18% [k] sys_close 0.14% [k] sys_munmap 0.13% [k] sys_poll Note that Oprofile uses the same method of __copy_user_inatomic() in arch/x86/oprofile/backtrace.c, but i believe that code is broken - i doubt the call-chain support for user-space stacks ever worked in oprofile - with perfcounters i can make this method crash under load. (we re-enter the NMI which due to IST executes over the exact same, still pending NMI frame. Kaboom.) I saw you being involved with the Oprofile code 3 years ago: | commit c34d1b4d165c67b966bca4aba026443d7ff161eb | Author: Hugh Dickins <hugh@xxxxxxxxxxx> | Date: Sat Oct 29 18:16:32 2005 -0700 | | [PATCH] mm: kill check_user_page_readable That method of __copy_user_inatomic(), while elegant, is subtly wrong in an NMI context. We really must avoid taking faults there. Ingo ------------> >From 3dfabc74c65904c9e6cf952391312d16ea772ef5 Mon Sep 17 00:00:00 2001 From: Ingo Molnar <mingo@xxxxxxx> Date: Mon, 15 Jun 2009 11:24:38 +0200 Subject: [PATCH] perf report: Add per system call overhead histogram Take advantage of call-graph percounter sampling/recording to display a non-trivial histogram: the true, collapsed/summarized cost measurement, on a per system call total overhead basis: aldebaran:~/linux/linux/tools/perf> ./perf record -g -a -f ~/hackbench 10 aldebaran:~/linux/linux/tools/perf> ./perf report -s symbol --syscalls | head -10 # # (3536 samples) # # Overhead Symbol # ........ ...... # 40.75% [k] sys_write 40.21% [k] sys_read 4.44% [k] do_nmi ... This is done by accounting each (reliable) call-chain that chains back to a given system call to that system call function. [ So in the above example we can see that hackbench spends about 40% of its total time somewhere in sys_write() and 40% somewhere in sys_read(), the rest of the time is spent in user-space. The time is not spent in sys_write() _itself_ but in one of its many child functions. ] Or, a recording of a (source files are already in the page-cache) kernel build: $ perf record -g -m 512 -f -- make -j32 kernel $ perf report -s s --syscalls | grep '\[k\]' | grep -v nmi 4.14% [k] do_page_fault 1.20% [k] sys_write 1.10% [k] sys_open 0.63% [k] sys_exit_group 0.48% [k] smp_apic_timer_interrupt 0.37% [k] sys_read 0.37% [k] sys_execve 0.20% [k] sys_mmap 0.18% [k] sys_close 0.14% [k] sys_munmap 0.13% [k] sys_poll 0.09% [k] sys_newstat 0.07% [k] sys_clone 0.06% [k] sys_newfstat 0.05% [k] sys_access 0.05% [k] schedule Shows the true total cost of each syscall variant that gets used during a kernel build. This profile reveals it that pagefaults are the costliest, followed by read()/write(). An interesting detail: timer interrupts cost 0.5% - or 0.5 seconds per 100 seconds of kernel build-time. (this was done with HZ=1000) The summary is done in 'perf report', i.e. in the post-processing stage - so once we have a good call-graph recording, this type of non-trivial high-level analysis becomes possible. Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> Cc: Mike Galbraith <efault@xxxxxx> Cc: Paul Mackerras <paulus@xxxxxxxxx> Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx> Cc: Pekka Enberg <penberg@xxxxxxxxxxxxxx> LKML-Reference: <new-submission> Signed-off-by: Ingo Molnar <mingo@xxxxxxx> --- tools/perf/builtin-report.c | 12 ++++++++++++ 1 files changed, 12 insertions(+), 0 deletions(-) diff --git a/tools/perf/builtin-report.c b/tools/perf/builtin-report.c index aebba56..1e2f5dd 100644 --- a/tools/perf/builtin-report.c +++ b/tools/perf/builtin-report.c @@ -40,6 +40,7 @@ static int dump_trace = 0; static int verbose; static int full_paths; +static int collapse_syscalls; static unsigned long page_size; static unsigned long mmap_window = 32; @@ -983,6 +984,15 @@ process_overflow_event(event_t *event, unsigned long offset, unsigned long head) for (i = 0; i < chain->nr; i++) dprintf("..... %2d: %p\n", i, (void *)chain->ips[i]); } + if (collapse_syscalls) { + /* + * Find the all-but-last kernel entry + * amongst the call-chains - to get + * to the level of system calls: + */ + if (chain->kernel >= 2) + ip = chain->ips[chain->kernel-2]; + } } dprintf(" ... thread: %s:%d\n", thread->comm, thread->pid); @@ -1343,6 +1353,8 @@ static const struct option options[] = { "sort by key(s): pid, comm, dso, symbol. Default: pid,symbol"), OPT_BOOLEAN('P', "full-paths", &full_paths, "Don't shorten the pathnames taking into account the cwd"), + OPT_BOOLEAN('S', "syscalls", &collapse_syscalls, + "show per syscall summary overhead, using call graph"), OPT_END() }; -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html