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    SubjectRe: [RFC 12/12] x86/dumpstack: Optimize save_stack_trace


    On 2016年06月29日 20:43, Byungchul Park wrote:
    > On Mon, Jun 20, 2016 at 04:50:37PM +0900, byungchul.park wrote:
    >>> -----Original Message-----
    >>> From: xinhui [mailto:xinhui.pan@linux.vnet.ibm.com]
    >>> Sent: Monday, June 20, 2016 4:29 PM
    >>> To: Byungchul Park; peterz@infradead.org; mingo@kernel.org
    >>> Cc: linux-kernel@vger.kernel.org; npiggin@suse.de; walken@google.com;
    >>> ak@suse.de; tglx@inhelltoy.tec.linutronix.de
    >>> Subject: Re: [RFC 12/12] x86/dumpstack: Optimize save_stack_trace
    >>>
    >>>
    >>> On 2016年06月20日 12:55, Byungchul Park wrote:
    >>>> Currently, x86 implementation of save_stack_trace() is walking all stack
    >>>> region word by word regardless of what the trace->max_entries is.
    >>>> However, it's unnecessary to walk after already fulfilling caller's
    >>>> requirement, say, if trace->nr_entries >= trace->max_entries is true.
    >>>>
    >>>> For example, CONFIG_LOCKDEP_CROSSRELEASE implementation calls
    >>>> save_stack_trace() with max_entries = 5 frequently. I measured its
    >>>> overhead and printed its difference of sched_clock() with my QEMU x86
    >>>> machine.
    >>>>
    >>>> The latency was improved over 70% when trace->max_entries = 5.
    >>>>
    >>> [snip]
    >>>
    >>>> +static int save_stack_end(void *data)
    >>>> +{
    >>>> + struct stack_trace *trace = data;
    >>>> + return trace->nr_entries >= trace->max_entries;
    >>>> +}
    >>>> +
    >>>> static const struct stacktrace_ops save_stack_ops = {
    >>>> .stack = save_stack_stack,
    >>>> .address = save_stack_address,
    >>> then why not check the return value of ->address(), -1 indicate there is
    >>> no room to store any pointer.
    >>
    >> Hello,
    >>
    >> Indeed. It also looks good to me even though it has to propagate the condition
    >> between callback functions. I will modify it if it's better.
    >
    > Do you also think it would be better to make it propagate the result of
    > ->address() rather than add a new callback, say, end_walk?
    >
    It's up to you. In my opinion, end_walk is better for reading.

    >>
    >> Thank you.
    >> Byungchul
    >>
    >>>
    >>>> .walk_stack = print_context_stack,
    >>>> + .end_walk = save_stack_end,
    >>>> };
    >>>>
    >>>> static const struct stacktrace_ops save_stack_ops_nosched = {
    >>>>
    >

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