RE: "struct perf_sample_data" alignment
... > What I remember is that since perf_sample_data is fairly large, > unconditionally initializing the whole thing is *slow* (and > -fauto-var-init=zero will hurt here). That will hurt everywhere. I can also imagine it hiding bugs and making people shrink on-stack arrays to the point where they either overrun or cause an unexpected error or string truncation. Initialising to zero is a bad choice if the aim is to avoid leaking stack. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
Re: "struct perf_sample_data" alignment
* Alexei Starovoitov wrote:
> > This seems to be it... (completely untested)
> >
> > ---
> > diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> > index 3f7f89ea5e51..918a296d2ca2 100644
> > --- a/include/linux/perf_event.h
> > +++ b/include/linux/perf_event.h
> > @@ -1032,7 +1032,9 @@ struct perf_sample_data {
> > u64 cgroup;
> > u64 data_page_size;
> > u64 code_page_size;
> > -} cacheline_aligned;
> > +};
> > +
> > +typedef struct perf_sample_data perf_sample_data_t cacheline_aligned;
> >
> > /* default value for data source */
> > #define PERF_MEM_NA (PERF_MEM_S(OP, NA) |\
> > diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> > index b0c45d923f0f..f32c623abef6 100644
> > --- a/kernel/trace/bpf_trace.c
> > +++ b/kernel/trace/bpf_trace.c
> > @@ -923,7 +923,7 @@ __bpf_perf_event_output(struct pt_regs *regs, struct
> > bpf_map *map,
> > * bpf_perf_event_output
> > */
> > struct bpf_trace_sample_data {
> > - struct perf_sample_data sds[3];
> > + perf_sample_data_t sds[3];
>
> bpf side doesn't care about about cacheline aligned.
> No need to add new typedef just for that.
So this structure is not supposed to be exposed to any ABI anywhere.
I did a (non-exhaustive) search of tooling, and there doesn't appear
to be any accidental exposure.
The in-kernel ABI interaction appears to be the following:
- In __perf_event_header_size() we only use fields within
perf_sample_data to size the header. Alignment won't change any of
the output.
- Ditto in perf_event__id_header_size().
I.e. I think we should just zap it per the patch below (untested).
Thanks,
Ingo
>
diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 3f7f89ea5e51..d75e03ff31ea 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -1032,7 +1032,7 @@ struct perf_sample_data {
u64 cgroup;
u64 data_page_size;
u64 code_page_size;
-} cacheline_aligned;
+};
/* default value for data source */
#define PERF_MEM_NA (PERF_MEM_S(OP, NA) |\
Re: "struct perf_sample_data" alignment
On Fri, Mar 5, 2021 at 7:01 AM Peter Zijlstra wrote:
>
> On Fri, Mar 05, 2021 at 09:36:30AM +0100, Peter Zijlstra wrote:
> > On Thu, Mar 04, 2021 at 07:45:44PM -0800, Linus Torvalds wrote:
> > > That cacheline_aligned goes back many years, this is not new, it
> > > seems to come from back in 2014: commit 2565711fb7d7 ("perf: Improve
> > > the perf_sample_data struct layout").
> >
> > long time ago...
> >
> > > But it really seems entirely and utterly bogus. That cacheline
> > > alignment makes things *worse*, when the variables are on the local
> > > stack. The local stack is already going to be dirty and in the cache,
> > > and aligning those things isn't going to - and I quote from the code
> > > in that commend in that commit - "minimize the cachelines touched".
> > >
> > > Quite the reverse. It's just going to make the stack frame use *more*
> > > memory, and make any cacheline usage _worse_.
> >
> > IIRC there is more history here, but I can't seem to find references
> > just now.
> >
> > What I remember is that since perf_sample_data is fairly large,
> > unconditionally initializing the whole thing is *slow* (and
> > -fauto-var-init=zero will hurt here).
> >
> > So at some point I removed that full initialization and made sure we
> > only unconditionally touched the first few variables, which gave a
> > measurable speedup.
> >
> > Then things got messy again and the commit 2565711fb7d7 referenced above
> > was cleanup, to get back to that initial state.
> >
> > Now, you're right that __cacheline_aligned on on-stack (and this is
> > indeed mostly on-stack) is fairly tedious (there were a few patches
> > recently to reduce the amount of on-stack instances).
> >
> > I'll put it on the todo list, along with that hotplug stuff (which I
> > tried to fix but ended up with an even bigger mess). I suppose we can
> > try and not have the alignment for the on-stack instances while
> > preserving it for the few off-stack ones.
> >
> > Also; we're running on the NMI stack, and that's not typically hot.
>
> This seems to be it... (completely untested)
>
> ---
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index 3f7f89ea5e51..918a296d2ca2 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -1032,7 +1032,9 @@ struct perf_sample_data {
> u64 cgroup;
> u64 data_page_size;
> u64 code_page_size;
> -} cacheline_aligned;
> +};
> +
> +typedef struct perf_sample_data perf_sample_data_t cacheline_aligned;
>
> /* default value for data source */
> #define PERF_MEM_NA (PERF_MEM_S(OP, NA) |\
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index b0c45d923f0f..f32c623abef6 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -923,7 +923,7 @@ __bpf_perf_event_output(struct pt_regs *regs, struct
> bpf_map *map,
> * bpf_perf_event_output
> */
> struct bpf_trace_sample_data {
> - struct perf_sample_data sds[3];
> + perf_sample_data_t sds[3];
bpf side doesn't care about about cacheline aligned.
No need to add new typedef just for that.
Re: "struct perf_sample_data" alignment
On Fri, Mar 05, 2021 at 09:36:30AM +0100, Peter Zijlstra wrote:
> On Thu, Mar 04, 2021 at 07:45:44PM -0800, Linus Torvalds wrote:
> > That cacheline_aligned goes back many years, this is not new, it
> > seems to come from back in 2014: commit 2565711fb7d7 ("perf: Improve
> > the perf_sample_data struct layout").
>
> long time ago...
>
> > But it really seems entirely and utterly bogus. That cacheline
> > alignment makes things *worse*, when the variables are on the local
> > stack. The local stack is already going to be dirty and in the cache,
> > and aligning those things isn't going to - and I quote from the code
> > in that commend in that commit - "minimize the cachelines touched".
> >
> > Quite the reverse. It's just going to make the stack frame use *more*
> > memory, and make any cacheline usage _worse_.
>
> IIRC there is more history here, but I can't seem to find references
> just now.
>
> What I remember is that since perf_sample_data is fairly large,
> unconditionally initializing the whole thing is *slow* (and
> -fauto-var-init=zero will hurt here).
>
> So at some point I removed that full initialization and made sure we
> only unconditionally touched the first few variables, which gave a
> measurable speedup.
>
> Then things got messy again and the commit 2565711fb7d7 referenced above
> was cleanup, to get back to that initial state.
>
> Now, you're right that __cacheline_aligned on on-stack (and this is
> indeed mostly on-stack) is fairly tedious (there were a few patches
> recently to reduce the amount of on-stack instances).
>
> I'll put it on the todo list, along with that hotplug stuff (which I
> tried to fix but ended up with an even bigger mess). I suppose we can
> try and not have the alignment for the on-stack instances while
> preserving it for the few off-stack ones.
>
> Also; we're running on the NMI stack, and that's not typically hot.
This seems to be it... (completely untested)
---
diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 3f7f89ea5e51..918a296d2ca2 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -1032,7 +1032,9 @@ struct perf_sample_data {
u64 cgroup;
u64 data_page_size;
u64 code_page_size;
-} cacheline_aligned;
+};
+
+typedef struct perf_sample_data perf_sample_data_t cacheline_aligned;
/* default value for data source */
#define PERF_MEM_NA (PERF_MEM_S(OP, NA) |\
diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index b0c45d923f0f..f32c623abef6 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -923,7 +923,7 @@ __bpf_perf_event_output(struct pt_regs *regs, struct
bpf_map *map,
* bpf_perf_event_output
*/
struct bpf_trace_sample_data {
- struct perf_sample_data sds[3];
+ perf_sample_data_t sds[3];
};
static DEFINE_PER_CPU(struct bpf_trace_sample_data, bpf_trace_sds);
Re: "struct perf_sample_data" alignment
On Thu, Mar 04, 2021 at 07:45:44PM -0800, Linus Torvalds wrote:
> That cacheline_aligned goes back many years, this is not new, it
> seems to come from back in 2014: commit 2565711fb7d7 ("perf: Improve
> the perf_sample_data struct layout").
long time ago...
> But it really seems entirely and utterly bogus. That cacheline
> alignment makes things *worse*, when the variables are on the local
> stack. The local stack is already going to be dirty and in the cache,
> and aligning those things isn't going to - and I quote from the code
> in that commend in that commit - "minimize the cachelines touched".
>
> Quite the reverse. It's just going to make the stack frame use *more*
> memory, and make any cacheline usage _worse_.
IIRC there is more history here, but I can't seem to find references
just now.
What I remember is that since perf_sample_data is fairly large,
unconditionally initializing the whole thing is *slow* (and
-fauto-var-init=zero will hurt here).
So at some point I removed that full initialization and made sure we
only unconditionally touched the first few variables, which gave a
measurable speedup.
Then things got messy again and the commit 2565711fb7d7 referenced above
was cleanup, to get back to that initial state.
Now, you're right that __cacheline_aligned on on-stack (and this is
indeed mostly on-stack) is fairly tedious (there were a few patches
recently to reduce the amount of on-stack instances).
I'll put it on the todo list, along with that hotplug stuff (which I
tried to fix but ended up with an even bigger mess). I suppose we can
try and not have the alignment for the on-stack instances while
preserving it for the few off-stack ones.
Also; we're running on the NMI stack, and that's not typically hot.
