Messages in this thread | | | Date | Wed, 16 Jan 2008 12:49:23 -0500 | From | Mathieu Desnoyers <> | Subject | Re: [RFC PATCH 16/22 -v2] add get_monotonic_cycles |
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* Mathieu Desnoyers (mathieu.desnoyers@polymtl.ca) wrote: > * Steven Rostedt (rostedt@goodmis.org) wrote: > > .... > > > > > One thing I want to clear up. The major difference between this > > latency_tracer and LTTng is what we consider fast paths. The latency > > tracer is recording things like enabling and disabling interrupts, preempt > > count changes, or simply profiling all function calls. Those are what I > > consider fast paths. The slow path WRT the latency_tracer are things like > > context switches. This is why I don't have a problem with copying the > > comm at context switch time. Because that _is_ a slow path for the latency > > tracer. > > LTTng hooks in the lockdep tracer to trace irq on/off, spinlocks, etc.. > in flight recorder mode, we have nothing to write to disk and therefore > we can handle very frequent events. We then do the analysis off-line > using the last MB written in the buffers. The advantage is that the > kernel dumbly writes data to a buffer : we therefore move the complexity > to user-space. > > I agree that some kind of tracing, like the one you are doing, might be > done more efficiently if you do a first clever analysis phase directly > in the kernel without writing the raw high event rate data in memory > buffers. However, I believe that it would be more powerful if we combine > the two approaches rather than trying to do everything in or out of the > kernel. LTTng could provide the comm names, priorities, etc, and your > tracer could provide the top X list of processes that had a bad > behavior. It would mean that the complete overall information would be > made available after a post-processing phase done in an analysis tool > like LTTV, but I don't see any problem with it. >
Just to expand a bit on the design-side of my proposal :
Your module would create "profiles" based on the hooks called. If we take the interrupt on/off for example, it would be called by lockdep and could keep a table of the top X instructions that disables interrupts for a long time. (it's just an example, you could want to save the pid instead...)
Then, whenever a "profile dump" is triggered, you would simply have to send the current state of your profile to lttng with something like :
struct irq_latency_table { void *ip; cycles_t time; };
/* * Make sure only one profile at a time is written to the trace for the * whole system. */ static DECLARE_MUTEX(latency_profile_mutex); static struct irq_latency_table latency_table[NR_ENTRIES];
void irq_latency_dump_profile(void) { int i; char namebuf[KSYM_NAME_LEN];
mutex_lock(&latency_profile_mutex); trace_mark(irq_latency_dump_begin, MARK_NOARGS); for (i = 0; i < NR_ENTRIES; i++) { sprint_symbol(namebuf, (unsigned long)latency_table[i].ip); trace_mark(irq_latency_entry, "ip %p symbol %s time %llu", latency_table[i].ip, namebuf, (unsigned long long)latency_table[i].time); } trace_mark(irq_latency_dump_end, MARK_NOARGS); mutex_unlock(&latency_profile_mutex); }
You can then create a LTTV module that will format your nice output each time a profile dump is encountered.
By doing this, your specialized profile generator would only have to hook into the irq on/off events to gather the information it needs, nothing more. I think that would trim the code size and the complexity of your module by an interesting factor.
Note that I could optimize the way I currently deal with symbols by not having to dump them in the trace, but since it's only for low rate events, this optimization has a low priority on my todo list.
Mathieu
-- Mathieu Desnoyers Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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