Re: [PATCH v7 bpf-next 3/3] selftests/bpf: add test for bpf_get_branch_snapshot

[Song Liu]

> On Sep 10, 2021, at 11:58 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 
> On Fri, Sep 10, 2021 at 11:33:52AM -0700, Song Liu wrote:
>> +	/* Given we stop LBR in software, we will waste a few entries.
>> +	 * But we should try to waste as few as possible entries. We are at
>> +	 * about 7 on x86_64 systems.
>> +	 * Add a check for < 10 so that we get heads-up when something
>> +	 * changes and wastes too many entries.
>> +	 */
>> +	ASSERT_LT(skel->bss->wasted_entries, 10, "check_wasted_entries");
> 
> It might be worth pointing out that you can easily bust this limit by
> enabling all the various tracepoints that are still in that code, but
> that that isn't a hard error since that's not the expected use case.
> 
> For example there's the wrmsr tracepoint that will inject 6 or so
> branches on top of that you now have. And I also think there's a
> tracepoint in local_irq_save() that can trigger.

Right. I did some test with a lot of debug config enabled. We do see a lot
more wasted entries there. 

Thanks,
Song