From: Jiri Olsa <olsajiri@gmail.com>
To: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>,
Matthew Wilcox <willy@infradead.org>,
Ian Rogers <irogers@google.com>,
Alexei Starovoitov <ast@kernel.org>,
Andrii Nakryiko <andrii@kernel.org>, Hao Luo <haoluo@google.com>,
Andrew Morton <akpm@linux-foundation.org>,
Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@redhat.com>,
Arnaldo Carvalho de Melo <acme@kernel.org>,
bpf@vger.kernel.org, linux-mm@kvack.org,
linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
linux-perf-users@vger.kernel.org, Martin KaFai Lau <kafai@fb.com>,
Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
John Fastabend <john.fastabend@gmail.com>,
KP Singh <kpsingh@chromium.org>,
Stanislav Fomichev <sdf@google.com>,
Daniel Borkmann <daniel@iogearbox.net>,
Namhyung Kim <namhyung@gmail.com>,
Dave Chinner <david@fromorbit.com>,
kernel-team@meta.com
Subject: Re: [PATCHv3 bpf-next 0/9] mm/bpf/perf: Store build id in file object
Date: Sat, 18 Mar 2023 09:34:05 +0100 [thread overview]
Message-ID: <ZBV3fRGcymXjcuRr@krava> (raw)
In-Reply-To: <CAEf4BzYQ-bktO9s8yhBk7xUoz=2NFrgdGviWsN2=HWPBaGv6hA@mail.gmail.com>
On Fri, Mar 17, 2023 at 11:08:44PM -0700, Andrii Nakryiko wrote:
SNIP
> > > That does depend upon the load, obviously, but it's not hard to collect -
> > > you already have more than enough hooks inserted in the relevant places.
> > > That might give a better appreciation of the reactions...
> >
> > One possibility would be a bit stolen from inode flags + hash keyed by
> > struct inode address (middle bits make for a decent hash function);
> > inode eviction would check that bit and kick the corresponding thing
> > from hash if the bit is set.
> >
> > Associating that thing with inode => hash lookup/insert + set the bit.
>
> This is an interesting idea, but now we are running into a few
> unnecessary problems. We need to have a global dynamically sized hash
> map in the system. If we fix the number of buckets, we risk either
> wasting memory on an underutilized system (if we oversize), or
> performance problems due to collisions (if we undersize) if we have a
> busy system with lots of executables mapped in memory. If we don't
> pre-size, then we are talking about reallocations, rehashing, and
> doing that under global lock or something like that. Further, we'd
> have to take locks on buckets, which causes further problems for
> looking up build ID from this hashmap in NMI context for perf events
> and BPF programs, as locks can't be safely taken under those
> conditions, and thus fetching build ID would still be unreliable
> (though less so than it is today, of course).
>
> All of this is solvable to some degree (but not perfectly and not with
> simple and elegant approaches), but seems like an unnecessarily
> overcomplication compared to the amount of memory that we hope to
> save. It still feels like a Kconfig-guarded 8 byte field per struct
> file is a reasonable price for gaining reliable build ID information
> for profiling/tracing tools.
>
>
> [0] https://drgn.readthedocs.io/en/latest/index.html
>
> [1] Script I used:
>
> from drgn.helpers.linux.pid import for_each_task
> from drgn.helpers.linux.fs import for_each_file
>
> task_cnt = 0
> file_set = set()
>
> for task in for_each_task(prog):
> task_cnt += 1
> try:
> for (fd, file) in for_each_file(task):
> file_set.add(file.value_())
> except:
> pass
>
> uniq_file_cnt = len(file_set)
> print(f"task_cnt={task_cnt} uniq_file_cnt={uniq_file_cnt}")
great you beat me to this, I wouldn't have thought of using drgn for this ;-)
I'll see if I can install it to some of our test servers
thanks,
jirka
next prev parent reply other threads:[~2023-03-18 8:34 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-03-16 17:01 [PATCHv3 bpf-next 0/9] mm/bpf/perf: Store build id in file object Jiri Olsa
2023-03-16 17:01 ` [PATCHv3 bpf-next 1/9] mm: " Jiri Olsa
2023-03-16 22:07 ` Andrii Nakryiko
2023-03-16 17:01 ` [PATCHv3 bpf-next 2/9] perf: Use file object build id in perf_event_mmap_event Jiri Olsa
2023-03-16 17:01 ` [PATCHv3 bpf-next 3/9] bpf: Use file object build id in stackmap Jiri Olsa
2023-03-16 22:07 ` Andrii Nakryiko
2023-03-16 17:01 ` [PATCHv3 bpf-next 4/9] bpf: Switch BUILD_ID_SIZE_MAX to enum Jiri Olsa
2023-03-16 22:07 ` Andrii Nakryiko
2023-03-16 17:01 ` [PATCHv3 bpf-next 5/9] selftests/bpf: Add read_buildid function Jiri Olsa
2023-03-16 22:23 ` Andrii Nakryiko
2023-03-30 22:05 ` Jiri Olsa
2023-03-16 17:01 ` [PATCHv3 bpf-next 6/9] selftests/bpf: Add err.h header Jiri Olsa
2023-03-16 22:24 ` Andrii Nakryiko
2023-03-16 17:01 ` [PATCHv3 bpf-next 7/9] selftests/bpf: Replace extract_build_id with read_build_id Jiri Olsa
2023-03-16 17:01 ` [PATCHv3 bpf-next 8/9] selftests/bpf: Add iter_task_vma_buildid test Jiri Olsa
2023-03-16 22:31 ` Andrii Nakryiko
2023-03-16 17:01 ` [PATCHv3 bpf-next 9/9] selftests/bpf: Add file_build_id test Jiri Olsa
2023-03-16 22:36 ` Andrii Nakryiko
2023-03-16 17:34 ` [PATCHv3 bpf-next 0/9] mm/bpf/perf: Store build id in file object Matthew Wilcox
2023-03-16 17:50 ` Ian Rogers
2023-03-16 21:51 ` Andrii Nakryiko
2023-03-17 3:51 ` Matthew Wilcox
2023-03-17 16:33 ` Andrii Nakryiko
2023-03-17 21:14 ` Al Viro
2023-03-17 21:21 ` Al Viro
2023-03-18 6:08 ` Andrii Nakryiko
2023-03-18 8:34 ` Jiri Olsa [this message]
2023-03-18 8:33 ` Jiri Olsa
2023-03-18 15:16 ` Matthew Wilcox
2023-03-18 17:40 ` Jiri Olsa
2023-03-22 15:45 ` Arnaldo Carvalho de Melo
2023-03-31 18:19 ` Andrii Nakryiko
2023-03-31 18:36 ` Matthew Wilcox
2023-03-31 20:27 ` Andrii Nakryiko
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