Re: [PATCH v2 2/2] mm: vmscan: add PIDs to vmscan tracepoints

[Steven Rostedt <rostedt@goodmis.org>]

On Tue, 16 Dec 2025 06:02:52 -0800
Thomas Ballasi <tballasi@linux.microsoft.com> wrote:

> The changes aims at adding additionnal tracepoints variables to help
> debuggers attribute them to specific processes.
> 
> The PID field uses in_task() to reliably detect when we're in process
> context and can safely access current->pid.  When not in process
> context (such as in interrupt or in an asynchronous RCU context), the
> field is set to -1 as a sentinel value.
> 
> Signed-off-by: Thomas Ballasi <tballasi@linux.microsoft.com>

Is this really needed? The trace events already show if you are in
interrupt context or not.

# tracer: nop
#
# entries-in-buffer/entries-written: 25817/25817   #P:8
#
#                                _-----=> irqs-off/BH-disabled
#                               / _----=> need-resched
#                              | / _---=> hardirq/softirq   <<<<------ Shows irq context
#                              || / _--=> preempt-depth
#                              ||| / _-=> migrate-disable
#                              |||| /     delay
#           TASK-PID     CPU#  |||||  TIMESTAMP  FUNCTION
#              | |         |   |||||     |         |
          <idle>-0       [002] d..1. 11429.293552: rcu_watching: Startirq 0 1 0x74c
          <idle>-0       [000] d.H1. 11429.293564: rcu_utilization: Start scheduler-tick
          <idle>-0       [000] d.H1. 11429.293566: rcu_utilization: End scheduler-tick
          <idle>-0       [002] dN.1. 11429.293567: rcu_watching: Endirq 1 0 0x74c
          <idle>-0       [002] dN.1. 11429.293568: rcu_watching: Start 0 1 0x754
          <idle>-0       [000] d.s1. 11429.293577: rcu_watching: --= 3 1 0xdf4
          <idle>-0       [002] dN.1. 11429.293579: rcu_utilization: Start context switch
          <idle>-0       [002] dN.1. 11429.293580: rcu_utilization: End context switch
       rcu_sched-15      [002] d..1. 11429.293589: rcu_grace_period: rcu_sched 132685 start
          <idle>-0       [000] dN.1. 11429.293592: rcu_watching: Endirq 1 0 0xdf4
       rcu_sched-15      [002] d..1. 11429.293592: rcu_grace_period: rcu_sched 132685 cpustart
       rcu_sched-15      [002] d..1. 11429.293592: rcu_grace_period_init: rcu_sched 132685 0 0 7 ff
          <idle>-0       [000] dN.1. 11429.293593: rcu_watching: Start 0 1 0xdfc

Thus, you can already tell if you are in interrupt context or not, and you
always get the current pid. The 'H', 'h' or 's' means you are in a
interrupt type context. ('H' for hard interrupt interrupting a softirq, 'h'
for just a hard interrupt, and 's' for a softirq).

What's the point of adding another field to cover the same information
that's already available?

-- Steve