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

[Mi, Dapeng]

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.

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. 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.