linux-kernel - Re: [PATCH] perf lock: Drop "-a" option from set of default arguments to cmd

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Date:	Wed, 12 May 2010 19:23:23 +0900
From:	Hitoshi Mitake <mitake@....info.waseda.ac.jp>
To:	Frederic Weisbecker <fweisbec@...il.com>
CC:	linux-kernel@...r.kernel.org, h.mitake@...il.com,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	Jason Baron <jbaron@...hat.com>,
	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
Subject: Re: [PATCH] perf lock: Drop "-a" option from set of default	arguments
 to cmd_record()

On 05/11/10 15:48, Frederic Weisbecker wrote:

 >>>
 >>> I think I'm going to unearth the injection code to reduce the size
 >>> of these events.
 >>>
 >>>
 >>
 >> Yeah, injection will be really helpful thing.
 >>
 >> And I have a rough idea for reducing event frequency.
 >>
 >> Many lock event sequences are like this form:
 >>   * acquire ->  acquired ->  release
 >>   * acquire ->  contended ->  acquired ->  release
 >> I think that making 3 or 4 events per each lock sequences
 >> is waste of CPU time and memory space.
 >>
 >> If threads store time of each events
 >> and make only 1 event at time of release,
 >> we will be able to reduce lots of time and space.
 >>
 >> For example, ID of each lock instance is 8 byte in x86_64.
 >> In this scheme 8 * 4 byte for ID will be only 8 byte.
 >> I think this optimization has worth to consider because of
 >> high frequency of lock events.
 >>
 >> How do you think?
 >
 >
 > You're right, we could optimize the lock events sequence layout.
 > What I'm afraid of is to break userspace, but ripping the name from
 > the lock events while introducing injection will break userspace 
anyway :-(

Really? For me, at least ripping the name with injection
doesn't make bad things for userspace.
What does the word "break" mean in this context?

 >
 > May be we can think about providing new lock events and announce the
 > deprecation of the old ones and remove them later. I'm not sure yet.
 >
 > But summing up in only one event is not possible. Having only one
 > lock_release event (and a lock init for name mapping) is suitable
 > for latency measurements only (timestamp + lock addr + wait time +
 > acquired time).
 > And once you dig into finer grained analysis like latency induced
 > by dependencies (take lock A and then take lock B under A, latency
 > of B depends of A), then you're screwed, because you only know
 > you've released locks at given times but you don't know when you
 > acquired them, hence you can't build a tree of dependencies with
 > sequences inside.

In my imagination, composing 3 or 4 events into one is meaning
timestamp of itself(it is also one of release) + lock addr
+ timestamp of acquire + timestamp of acquired
(+ timestamp of contended) + misc information
like flags.

I'd like to call this new event as "batch event" in below.

If perf lock reads one batch event, original events of 3 or 4
can be reconstructed in userspace.
So I think dependency relation between locks can be obtained
with sorting reconstructed events with timestamp.

For this way, each threads have to hold memory for
size of batch event * MAX_LOCK_DEPTH.

I'm not sure about possibility and effect of this way :(
and if I misunderstood something about your opinion, please correct me

 >
 > Ideally, I think that we need to remove lock contended and only
 > trigger lock_acquired when we contended. We obviously don't need
 > lock_contended/lock_acquired if the lock wasn't contended.

Yeah, lock events can be reduced more.

 >
 > We could even remove lock_acquire and only play with lock_acquired,
 > changing a bit the rules, but that will make us lose all the dependencies
 > "intra-lock". I mean there are locks which slowpath are implemented 
on top
 > of other locks: mutexes use mutex->wait_lock spinlock for example.
 >
 >

Do you mean that the relation like acquire(mutex) -> acquire(spinlock)
-> acquired(spinlock) -> acquired(mutex) -> release(spinlock)
will be lost?

It seems taht locks on other locks are only mutex and rwsem.
I think that we have a way to rewrite their lock events
of mutex and rwsem for intra-lock dependencies.

But I cannot measure the actual cost of it :(
So I cannot say easily this is possible...
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