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From: Song Liu <songliubraving@fb.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: bpf <bpf@vger.kernel.org>,
	open list <linux-kernel@vger.kernel.org>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Ingo Molnar <mingo@redhat.com>, Kajol Jain <kjain@linux.ibm.com>,
	Kernel Team <Kernel-team@fb.com>
Subject: Re: [PATCH v5 bpf-next 2/3] bpf: introduce helper bpf_get_branch_snapshot
Date: Fri, 3 Sep 2021 17:06:12 +0000	[thread overview]
Message-ID: <50D0C11D-71EF-4C82-A208-D7C73E241805@fb.com> (raw)
In-Reply-To: <YTHhOy1gqr44C1bI@hirez.programming.kicks-ass.net>



> On Sep 3, 2021, at 1:47 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> 
> On Thu, Sep 02, 2021 at 09:57:05AM -0700, Song Liu wrote:
>> +BPF_CALL_3(bpf_get_branch_snapshot, void *, buf, u32, size, u64, flags)
>> +{
>> +	static const u32 br_entry_size = sizeof(struct perf_branch_entry);
>> +	u32 entry_cnt = size / br_entry_size;
>> +
>> +	if (unlikely(flags))
>> +		return -EINVAL;
>> +
>> +	if (!buf || (size % br_entry_size != 0))
>> +		return -EINVAL;
>> +
>> +	entry_cnt = static_call(perf_snapshot_branch_stack)(buf, entry_cnt);
> 
> That's at least 2, possibly 3 branches just from the sanity checks, plus
> at least one from starting the BPF prog and one from calling this
> function, gets you at ~5 branch entries gone before you even do the
> snapshot thing.

Let me try to shuffle the function and get rid of some of these checks. 

> 
> Less if you're in branch-stack mode.
> 
> Can't the validator help with getting rid of the some of that?
> 
> I suppose you have to have this helper function because the JIT cannot
> emit static_call()... although in this case one could cheat and simply
> emit a call to static_call_query() and not bother with dynamic updates
> (because there aren't any).

We only JIT some key helper functions. I didn't think about that because 
current version is OK for mainstream and future hardware. I guess we 
can try JIT if it turns out some architecture needs more optimization. 

Thanks,
Song


  reply	other threads:[~2021-09-03 17:06 UTC|newest]

Thread overview: 27+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-02 16:57 [PATCH v5 bpf-next 0/3] bpf: introduce bpf_get_branch_snapshot Song Liu
2021-09-02 16:57 ` [PATCH v5 bpf-next 1/3] perf: enable branch record for software events Song Liu
2021-09-02 20:49   ` John Fastabend
2021-09-03  8:02   ` Peter Zijlstra
2021-09-03 16:50     ` Song Liu
2021-09-07 18:59       ` Song Liu
2021-09-07 20:50         ` Andrii Nakryiko
2021-09-03  8:27   ` Peter Zijlstra
2021-09-03 16:45     ` Song Liu
2021-09-04 10:18       ` Peter Zijlstra
2021-09-02 16:57 ` [PATCH v5 bpf-next 2/3] bpf: introduce helper bpf_get_branch_snapshot Song Liu
2021-09-02 20:56   ` John Fastabend
2021-09-02 22:04     ` Song Liu
2021-09-02 22:53   ` Andrii Nakryiko
2021-09-02 23:03     ` Song Liu
2021-09-02 23:05       ` Andrii Nakryiko
2021-09-03  1:03   ` Alexei Starovoitov
2021-09-03  8:28   ` Peter Zijlstra
2021-09-03 16:58     ` Song Liu
2021-09-03  8:47   ` Peter Zijlstra
2021-09-03 17:06     ` Song Liu [this message]
2021-09-03 17:10     ` Andrii Nakryiko
2021-09-04 10:24       ` Peter Zijlstra
2021-09-04 10:55         ` Peter Zijlstra
2021-09-02 16:57 ` [PATCH v5 bpf-next 3/3] selftests/bpf: add test for bpf_get_branch_snapshot Song Liu
2021-09-02 21:05   ` John Fastabend
2021-09-02 22:54 ` [PATCH v5 bpf-next 0/3] bpf: introduce bpf_get_branch_snapshot Andrii Nakryiko

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