Re: [PATCH] perf record: mmap output file - v2

[David Ahern]

On 10/15/13 1:31 AM, Namhyung Kim wrote:
> Hi David,
>
> On Mon, 14 Oct 2013 20:55:31 -0600, David Ahern wrote:
> > When recording raw_syscalls for the entire system, e.g.,
> >      perf record -e raw_syscalls:*,sched:sched_switch -a -- sleep 1
> >
> > you end up with a negative feedback loop as perf itself calls
> > write() fairly often. This patch handles the problem by mmap'ing the
> > file in chunks of 64M at a time and copies events from the event buffers
> > to the file avoiding write system calls.
> >
> > Before (with write syscall):
> >
> > perf record -o /tmp/perf.data -e raw_syscalls:*,sched:sched_switch -a -- sleep 1
> > [ perf record: Woken up 0 times to write data ]
> > [ perf record: Captured and wrote 81.843 MB /tmp/perf.data (~3575786 samples) ]
> >
> > After (using mmap):
> >
> > perf record -o /tmp/perf.data -e raw_syscalls:*,sched:sched_switch -a -- sleep 1
> > [ perf record: Woken up 31 times to write data ]
> > [ perf record: Captured and wrote 8.203 MB /tmp/perf.data (~358388 samples) ]
>
> Why do they have that different size?

perf calls write() for each mmap, each time through the loop. Each write 
generates 2 events (syscall entry + exit) -- ie., generates more events. 
That's the negative feedback loop.


> [SNIP]
> > +
> > +		rec->mmap_addr = mmap(NULL, rec->mmap_size,
> > +				     PROT_WRITE | PROT_READ,
> > +				     MAP_SHARED,
> > +				     rec->output,
> > +				     offset);
> > +
> > +		if (rec->mmap_addr == MAP_FAILED) {
> > +			pr_err("mmap failed: %d: %s\n", errno, strerror(errno));
> > +			return -1;
> > +		}
> > +
> > +		/* expand file to include this mmap segment */
> > +		if (ftruncate(rec->output, offset + rec->mmap_size) != 0) {
> > +			pr_err("ftruncate failed\n");
> > +			return -1;
> > +		}
>
> I think this mmap + ftruncate should be reordered.  Although it looks
> work without problems the mmap man pages says it's unspecified behavior.
>
>         A file is mapped in multiples of the page size.  For a file that is not
>         a multiple of the page  size,  the  remaining  memory  is  zeroed  when
>         mapped, and writes to that region are not written out to the file.  The
>         effect of changing the size of the underlying file of a mapping on  the
>         pages  that  correspond  to  added  or  removed  regions of the file is
>         unspecified.

The mmap only expands the address range; the ftruncate expands the file 
behind the mmap. Both are needed and must succeed to function properly, 
and I don't see how the order matters. ie.,

This order has an extra call on the failure path:
  ftruncate
  mmap
  - on failure call ftruncate to reset file size

The order I have does not have that problem:
  mmap
  ftruncate

Here on failure just return -1 and we end the session.

David
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