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Date: Sun, 12 Oct 2008 23:10:15 +0300 From: Török Edwin <edwintorok@...il.com> To: Peter Zijlstra <a.p.zijlstra@...llo.nl> CC: mingo@...e.hu, sandmann@...mi.au.dk, linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>, Arjan van de Ven <arjan@...radead.org> Subject: Re: [PATCH 4/4] Implement semaphore latency tracer On 2008-10-12 22:13, Peter Zijlstra wrote: > On Sun, 2008-10-12 at 16:12 +0300, Török Edwin wrote: > >> Each time a down_read or down_write fails, a unique latency id is generated. >> Later when someone releases the semaphore, it is blamed for the latency of all >> tasks on the wait_list of the semaphore. >> If you would group the output from latency_trace by the latency_id you get all those >> who were contending on a lock, and the tasks that were holding the lock. >> An entry in latency_trace has the format: >> (latency_id) [semaphore_id] read|write >> stacktrace <= stacktrace >> > > What can this tracer do that latencytop cannot already do? Latencytop can show latencies in down_read or down_write (and is very useful at doing that), but it cannot show who else was holding the semaphore, i.e. the true cause of the latency. Consider: process A holds a semaphore for reading, process B tries to acquire it for writing and fails. Latencytop shows the latency in process B, but doesn't show anything about process A. The semlat tracer is doing something more similar to lockstat, but doesn't need lockdep, and it adds tracepoints on the slowpath only (lock failed, wakeup). Best regards, --Edwin -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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