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Re: "lttng comparison with other tracers" page
- From: fche at redhat dot com (Frank Ch. Eigler)
- To: Mathieu Desnoyers <compudj at krystal dot dyndns dot org>
- Cc: Josh Stone <jistone at redhat dot com>, systemtap at sources dot redhat dot com, pierre-marc dot fournier at polymtl dot ca, michel dot dagenais at polymtl dot ca
- Date: Wed, 18 Mar 2009 10:58:16 -0400
- Subject: Re: "lttng comparison with other tracers" page
- References: <20090312212907.GB17039@redhat.com> <20090312223036.GA30346@Krystal> <20090312232557.GA16297@redhat.com> <20090314025858.GD22526@Krystal> <20090314122841.GB12974@redhat.com> <20090314145053.GA8830@Krystal> <20090317190243.GH12974@redhat.com> <20090317204131.GA19431@Krystal> <20090317210530.GJ12974@redhat.com> <49C049D2.4020503@redhat.com> <20090318011920.GA26367@Krystal>
Mathieu Desnoyers <compudj@krystal.dyndns.org> writes:
> [...]
>> I looked at it for a while, but I couldn't determine anything more than
>> simple interrupt-reentrance. The above tracepoints are hit often enough
>> that even a low-rate of reentrance will add up quickly.
>>
>> I did add a "-DINTERRUPTIBLE=0" option to disable interrupts during our
>> handlers, which eliminated the problem as far as I could see.
>
> Hrm, the systemtap code should always expect reentrancy coming from
> interrupts. The way to deal with this is either to _always_ disable
> interrupts around the probe code,
This is possible, but does negatively impact overall
latency/performance, so we no longer do it by default. There is a
gross knob (-DINTERRUPTIBLE=0) but we could make it more fine-grained.
> or to use atomic operations within the probe and allow nested probe
> execution at any given point of the probe.
This may work for a plain tracer that carries no state from probe to
probe, but in our case a script may possess locks on global variables
and be mid-computation, so complete reentrancy is not possible.
In our case, the simplest solution may be to raise or eliminate the
skipped probe limit.
- FChE