Re: [PATCH 2/2] perf/x86: Disable precise sampling for PERF_SAMPLE_STACK_USER

[Peter Zijlstra]

On Tue, Sep 08, 2026 at 04:56:22PM +0800, Mi, Dapeng wrote:
> 
> On 9/8/2026 4:49 PM, Peter Zijlstra wrote:
> > On Tue, Sep 08, 2026 at 03:51:02PM +0800, Dapeng Mi wrote:
> >> PERF_SAMPLE_STACK_USER needs to return the user stack and user registers
> >> to user space when the PMI exits. Since the skid from the PEBS/IBS sample
> >> and PMI delivery, the PEBS/IBS register snapshot (especially IP/SP/BP)
> >> can diverge from the user stack at PMI return. That mismatch breaks DWARF
> >> unwinding.
> >>
> >> Precise sampling provides no benefit in this case, so disable PEBS/IBS
> >> precise sampling and allow only PMI-based sampling when
> >> PERF_SAMPLE_STACK_USER is requested.
> >>
> >> Reported-by: Gennady Kupava <gennady.kupava@xxxxxxxxx>
> >> Closes: https://lore.kernel.org/all/CAPu-DQqF0aF6=GS8Z6KKWeeX_V5LiXeKU_rJQZC+uGg8zuTPNw@xxxxxxxxxxxxxx/
> >> Cc: Ravi Bangoria <ravi.bangoria@xxxxxxx>
> >> Fixes: c5ebcedb566e ("perf: Add ability to attach user stack dump to sample")
> >> Signed-off-by: Dapeng Mi <dapeng1.mi@xxxxxxxxxxxxxxx>
> > This breaks long standing existing behaviour.
> 
> Yeah, but it seems there is no better way to fix this issue.

Breaking things that worked before isn't fixing.. people get upset.

> An alternative way to fix this issue is still to return the PMI
> context register state rather than the PEBS precise registers for user
> stack sampling, but this actually falls back the imprecise PMI-based
> sampling. 

That's what we already do, no? I have distinct memories of making the
stack unwind use the NMI regs rather then the PEBS regs.

> In my opinion, it could even make the thing worse. User
> requires to get precise samplings, but perf silently returns imprecise
> records, this would mislead user. 

Mostly just the unwind might be off a little, the rest is accurate. This
has been the case 'forever'. Performance analysis isn't for silly
people, if they can't deal with a little fuzz then perhaps they're in
the wrong business.