On Thu, Apr 21, 2016 at 04:27:19PM -0700, Andy Lutomirski wrote:
> > Did that help? Or did I confuse you moar?
> >
>
> I think I'm starting to get it. What if we rearrange slightly, like this:
>
> perf_sample_data already has a struct perf_regs in it. We could add a
> flags field to the first chunk of perf_sample_data:
>
> u64 sample_flags;
I actually considered that for another problem. Didn't like it then, but
seeing how I still haven't figured out a better way and you're now
proposing this too, maybe...
Part of the problem is that this will completely exhaust that first
cacheline :/
> perf_sample_data_init sets sample_flags to zero.
And while we're on struct perf_sample_data, that thing has gotten
insanely large. We carry it on-stack!
It should be fairly easy to take regs_user_copy out and use a per-cpu
array of them things for this I think, see below.
> Now we rename perf_sample_regs_user to __perf_sample_regs_user and
> make it non-static. We also teach it to set do data->sample_flags |=
> PERF_SAMPLE_FLAGS_HAS_REGS_USER. We add:
>
> static void perf_fetch_regs_user(struct perf_sample_data *data, struct
> pt_regs *interrupt_regs)
> {
> if (data->sample_flags & PERF_SAMPLE_FLAGS_HAS_REGS_USER)
> return;
>
> __perf_sample_regs_user(&data->regs_user, interrupt_regs,
> &data->regs_user_copy);
> }
I meant to change perf_prepare_sample() to do:
u64 sample_type = event->attr.sample_type & ~data.sample_type;
or something similar, such that we can override/avoid some of the work
there.
> (Hmm. This only really works well if we can guarantee that
> interrupt_regs remains valid for the life of the perf_sample_data
> object. Could we perhaps move the interrupt_regs pointer *into*
> perf_sample_data and stop passing it all over the place?)
So the problem with that is that we'll now overflow the one cacheline,
and the last time I really looked at this that made samples that much
slower.
It might be time to re-evaluate this stuff, since pretty much everything
will eventually write into perf_sample_data::ip etc.. which is the
second line anyway.
Also, looking at it, we actually have a pointer in there for this,
perf_sample_data::regs_intr::regs, but its at the very tail of this
monster, 4 cachelines off the normal path.
> We change all the callers of perf_sample_regs_user to use
> perf_fetch_regs_user instead.
There's only the one site currently, but yeah.
> What do you think? If you like it, I can probably find some time to
> give it a shot, but I don't guarantee that I won't miss some subtlety
> in its interaction with the rest of the event output code.
Sure give it a go, I'll stomp on it to fix the pebs-time issue (we need
to skip perf_prepare_sample's PERF_SAMPLE_TIME branch for that).
> On a vaguely related note, why is the big prebs-to-pt_regs copy
> conditional on (sample_type & PERF_SAMPLE_REGS_INTR)? I bet it would
> be faster to make it unconditional, because you could avoid copying
> over the entire pt_regs struct if PERF_SAMPLE_REGS_INTR isn't set.
Hmm, yes.. that code did move about a bit, not sure what it looked like
originally.
In any case, That fully copy is overkill in the simple case as well, I
think that could get away with only copying cs,flags.
Compile tested only..
---
Subject: perf: Replace perf_sample_data::regs_user_copy with per-cpu storage
struct perf_sample_data is immense, and we carry it on stack, shrink it
some.
struct perf_sample_data {
/* size: 384, cachelines: 6, members: 19 */
}
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
include/linux/perf_event.h | 2 --
kernel/events/core.c | 23 +++++++++++++++++------
2 files changed, 17 insertions(+), 8 deletions(-)
diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 85749ae8cb5f..dd2cab6c5bbb 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -795,8 +795,6 @@ struct perf_sample_data {
* on arch details.
*/
struct perf_regs regs_user;
- struct pt_regs regs_user_copy;
-
struct perf_regs regs_intr;
u64 stack_user_size;
} ____cacheline_aligned;
diff --git a/kernel/events/core.c b/kernel/events/core.c
index eabeb2aec00f..72754607d2cd 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -5146,15 +5146,27 @@ perf_output_sample_regs(struct perf_output_handle *handle,
}
}
-static void perf_sample_regs_user(struct perf_regs *regs_user,
- struct pt_regs *regs,
- struct pt_regs *regs_user_copy)
+static DEFINE_PER_CPU(struct pt_regs, __regs_user[4]);
+
+static struct pt_regs *regs_user_ptr(void)
+{
+ if (in_nmi())
+ return this_cpu_ptr(&__regs_user[0]);
+ if (in_interrupt())
+ return this_cpu_ptr(&__regs_user[1]);
+ if (in_serving_softirq())
+ return this_cpu_ptr(&__regs_user[2]);
+ return this_cpu_ptr(&__regs_user[3]);
+}
+
+static void
+perf_sample_regs_user(struct perf_regs *regs_user, struct pt_regs *regs)
{
if (user_mode(regs)) {
regs_user->abi = perf_reg_abi(current);
regs_user->regs = regs;
} else if (current->mm) {
- perf_get_regs_user(regs_user, regs, regs_user_copy);
+ perf_get_regs_user(regs_user, regs, regs_user_ptr());
} else {
regs_user->abi = PERF_SAMPLE_REGS_ABI_NONE;
regs_user->regs = NULL;
@@ -5638,8 +5650,7 @@ void perf_prepare_sample(struct perf_event_header *header,
}
if (sample_type & (PERF_SAMPLE_REGS_USER | PERF_SAMPLE_STACK_USER))
- perf_sample_regs_user(&data->regs_user, regs,
- &data->regs_user_copy);
+ perf_sample_regs_user(&data->regs_user, regs);
if (sample_type & PERF_SAMPLE_REGS_USER) {
/* regs dump ABI info */
--
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