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    SubjectRe: [PATCH 2/6] pstore: Add event tracing support
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    On 9/22/2018 2:35 PM, Joel Fernandes wrote:
    > On Sat, Sep 8, 2018 at 4:28 PM Sai Prakash Ranjan
    > <saiprakash.ranjan@codeaurora.org> wrote:
    >>
    > Could you just split the pstore space into a per-cpu event buffer like
    > we are doing for ftrace-on-pstore? Then you don't need to lock. I fear
    > the lock contention will be apparent. The pstore code already has
    > plumbing to split the store buffer per CPU.
    >

    Hi Joel,

    Thanks a lot for reviewing.

    I just looked at per-cpu buffer for ftrace and itseems the pstore percpu
    records will need to be merged into one record if we add this support
    for events. Merging of ftrace logs is based on timestamp, but for events
    we do not have timestamp field (for this specific reason I have added
    timestamp field for IO event tracing so that atleast we can know the cpu
    number in pstore output). For example, the sched event pstore output
    below has no timestamp field, so how do we merge per-cpu logs?

    # tail /sys/fs/pstore/event-ramoops-0
    sched_waking: comm=rcu_sched pid=11 prio=120 target_cpu=002
    sched_wakeup: comm=rcu_sched pid=11 prio=120 target_cpu=002

    Also Pstore ftrace log format is fixed i.e.,(CPU:%d ts:%llu %08lx %08lx
    %pf <- %pF\n"), but different events will have different formats and
    we will not be able to add timestamp field like how pstore ftrace does
    using pstore_ftrace_write_timestamp() and pstore_ftrace_read_timestamp().

    Sorry if I am confusing you, I can explain better I guess.

    > Also I think this spinlock can be moved further down.
    >

    OK. Something like this would suffice?

    {{{
    spin_lock_irqsave(&psinfo->buf_lock, flags);

    record.buf = (char *)(seq->buffer);
    record.size = seq->len;
    psinfo->write(&record);

    spin_unlock_irqrestore(&psinfo->buf_lock, flags);
    }}}

    >> +
    >> + trace_seq_init(&iter->seq);
    >> + iter->ent = fbuffer->entry;
    >> + event_call->event.funcs->trace(iter, 0, event);
    >> + trace_seq_putc(&iter->seq, 0);
    >
    > Would it be possible to store the binary trace record in the pstore
    > buffer instead of outputting text? I suspect that will both be faster
    > and less space.
    >

    I will try this and come back.

    Thanks,
    Sai

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