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

On 9/8/2026 6:19 PM, Peter Zijlstra wrote:

On Tue, Sep 08, 2026 at 04:56:22PM +0800, Mi, Dapeng wrote:

quoted

On 9/8/2026 4:49 PM, Peter Zijlstra wrote:

quoted

On Tue, Sep 08, 2026 at 03:51:02PM +0800, Dapeng Mi wrote:

quoted

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 <redacted>
Closes: https://lore.kernel.org/all/CAPu-DQqF0aF6=GS8Z6KKWeeX_V5LiXeKU_rJQZC+uGg8zuTPNw@mail.gmail.com/ (local)
Cc: Ravi Bangoria <redacted>
Fixes: c5ebcedb566e ("perf: Add ability to attach user stack dump to sample")
Signed-off-by: Dapeng Mi <redacted>

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.

quoted

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.

Unfortunately it's not. :(

Currently pt_regs->ip would be unconditionally overwritten by PEBS/IBS
snapshotted IP register value, and then the pt_regs->ip is used to
generated the SAMPLE_IP.

    if (filtered_sample_type & PERF_SAMPLE_IP) {
        data->ip = perf_instruction_pointer(event, regs);
        data->sample_flags |= PERF_SAMPLE_IP;
    }

As Ian suggested, the better way to fix this issue could be to decouple
PERF_SAMPLE_IP and PERF_REG_X86_IP. PERF_SAMPLE_IP still stores the precise
IP from PEBS/IBS, but the whole user register snapshot keeps the PMI
context registers. DWARF depends on the user register snapshot to unwind
the call chain instead of PERF_SAMPLE_IP.

I would follow this way and send V2 patches. 

Thanks.

quoted

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.