linux-kernel - Re: [PATCH v2] trace: Add migrate-disabled counter to tracing output.

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Date:   Thu, 2 Sep 2021 09:16:50 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v2] trace: Add migrate-disabled counter to tracing output.

On 2021-08-10 15:26:26 [+0200], To Steven Rostedt wrote:
> From: Thomas Gleixner <tglx@...utronix.de>
> 
> migrate_disable() forbids task migration to another CPU. It is available
> since v5.11 and has already users such as highmem or BPF. It is useful
> to observe this task state in tracing which already has other states
> like the preemption counter.
> 
> Instead of adding the migrate disable counter as a new entry to struct
> trace_entry, which would extend the whole struct by four bytes, it is
> squashed into the preempt-disable counter. The lower four bits represent
> the preemption counter, the upper four bits represent the migrate
> disable counter. Both counter shouldn't exceed 15 but if they do, there
> is a safety net which caps the value at 15.
> 
> Add the migrate-disable counter to the trace entry so it shows up in the
> trace. Due to the users mentioned above, it is already possible to
> observe it:
> 
> |  bash-1108    [000] ...21    73.950578: rss_stat: mm_id=2213312838 curr=0 type=MM_ANONPAGES size=8192B
> |  bash-1108    [000] d..31    73.951222: irq_disable: caller=flush_tlb_mm_range+0x115/0x130 parent=ptep_clear_flush+0x42/0x50
> |  bash-1108    [000] d..31    73.951222: tlb_flush: pages:1 reason:local mm shootdown (3)
> 
> The last value is the migrate-disable counter.
> 
> Things that popped up:
> - trace_print_lat_context() does not print the migrate counter. Not sure
>   if it should. It is used in "verbose" mode and uses 8 digits and I'm
>   not sure ther is something processing the value.
> 
> - trace_define_common_fields() now defines a different variable. This
>   probably breaks things. No ide what to do in order to preserve the old
>   behaviour. Since this is used as a filter it should be split somehow
>   to be able to match both nibbles here.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> [bigeasy: patch description.]
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> ---
> v1…v2:
>   - use the upper nibble of the preemption counter for the migrate
>     disable counter and the lower for the preemption counter. This
>     avoids extending the struct by four bytes due to alignment /
>     padding.
 
 Ping.

Sebastian