lkml.org 
[lkml]   [2006]   [Jun]   [30]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    SubjectRe: [PATCH 10/17] 2.6.17.1 perfmon2 patch for review: PMU context switch
    From
    Date
    Stephane Eranian <eranian@hpl.hp.com> writes:

    > Andi,
    >
    > Thanks for your feedback. I will make the changes you
    > requested.
    >
    > About the context switch code, what about I do the following
    > in __switch_to():
    >
    > __kprobes struct task_struct *
    > __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
    > {
    > struct thread_struct *prev = &prev_p->thread,
    > *next = &next_p->thread;
    > int cpu = smp_processor_id();
    > struct tss_struct *tss = &per_cpu(init_tss, cpu);
    >
    > if (unlikely(__get_cpu_var(pmu_ctx) || next_p->pfm_context))
    > __pfm_ctxswout(prev_p, next_p);
    >
    > /*
    > * Reload esp0, LDT and the page table pointer:
    > */
    > tss->rsp0 = next->rsp0;
    >
    > There is now a single hook and a conditional branch.
    > this is similar to what you have with the debug registers.

    It's still more than there was before. Also __get_cpu_var
    is quite a lot of instructions.

    I would suggest you borrow some bits in one of the process
    or thread info flags and then do a single test

    if (unlikely(thr->flags & (DEBUG|PERFMON)) != 0) {
    if (flags & DEBUG)
    ... do debug ...
    if (flags & PERFMON)
    ... do perfmon ...
    }

    [which you're at it you can probably add ioports in there too -
    improving existing code is always a good thing]

    Ideally flags is in some cache line that is already
    touched during context switch. If not you might need
    to change the layout.

    It's ok to put the do_perfmon stuff into a separate noinline
    function because that will disturb the register allocation
    in the caller less.

    I would suggest doing this in separate preparing patches that
    first just do it for existing facilities.

    -Andi

    P.S.: My comments probably apply to the i386 versions too
    although I haven't read them.
    -
    To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
    the body of a message to majordomo@vger.kernel.org
    More majordomo info at http://vger.kernel.org/majordomo-info.html
    Please read the FAQ at http://www.tux.org/lkml/

    \
     
     \ /
      Last update: 2006-06-30 15:01    [W:0.031 / U:59.316 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site