linux-kernel - Re: [PATCH 0/34] Implement RCU Tasks Trace in terms of SRCU-fast and optimize

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Message-ID: <CAADnVQKNxGFOWN7-HmzObYobW2y33g-i3xsNSkKicx88hqe70w@mail.gmail.com>
Date: Wed, 24 Sep 2025 09:49:23 +0200
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: rcu@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>, 
	Kernel Team <kernel-team@...a.com>, Steven Rostedt <rostedt@...dmis.org>, 
	Andrii Nakryiko <andrii@...nel.org>, Alexei Starovoitov <ast@...nel.org>, 
	Peter Zijlstra <peterz@...radead.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH 0/34] Implement RCU Tasks Trace in terms of SRCU-fast and optimize

On Tue, Sep 23, 2025 at 4:21 PM Paul E. McKenney <paulmck@...nel.org> wrote:
>
> Hello!
>
> This series re-implements RCU Tasks Trace in terms of SRCU-fast,
> reducing the size of the Linux-kernel RCU implementation by several
> hundred lines of code.  It also removes a conditional branch from the
> srcu_read_lock_fast() implementation in order to make SRCU-fast a
> bit more fastpath-friendly.  The patches are as follows:
>
> 1.      Re-implement RCU Tasks Trace in terms of SRCU-fast.
>
> 2.      Remove unused ->trc_ipi_to_cpu and ->trc_blkd_cpu from
>         task_struct.
>
> 3.      Remove ->trc_blkd_node from task_struct.
>
> 4.      Remove ->trc_holdout_list from task_struct.
>
> 5.      Remove rcu_tasks_trace_qs() and the functions that it calls.
>
> 6.      context_tracking: Remove
>         rcu_task_trace_heavyweight_{enter,exit}().
>
> 7.      Remove ->trc_reader_special from task_struct.
>
> 8.      Remove now-empty RCU Tasks Trace functions and calls to them.
>
> 9.      Remove unused rcu_tasks_trace_lazy_ms and trc_stall_chk_rdr
>         struct.
>
> 10.     Remove now-empty show_rcu_tasks_trace_gp_kthread() function.
>
> 11.     Remove now-empty rcu_tasks_trace_get_gp_data() function.
>
> 12.     Remove now-empty rcu_tasks_trace_torture_stats_print() function.
>
> 13.     Remove now-empty get_rcu_tasks_trace_gp_kthread() function.
>
> 14.     Move rcu_tasks_trace_srcu_struct out of #ifdef
>         CONFIG_TASKS_RCU_GENERIC.
>
> 15.     Add noinstr-fast rcu_read_{,un}lock_tasks_trace() APIs.
>
> 16.     Remove now-unused rcu_task_ipi_delay and TASKS_TRACE_RCU_READ_MB.
>
> 17.     Create a DEFINE_SRCU_FAST().
>
> 18.     Use smp_mb() only when necessary in RCU Tasks Trace readers.
>
> 19.     Update Requirements.rst for RCU Tasks Trace.
>
> 20.     Deprecate rcu_read_{,un}lock_trace().
>
> 21.     Mark diagnostic functions as notrace.
>
> 22.     Guard __DECLARE_TRACE() use of __DO_TRACE_CALL() with SRCU-fast.
>
> 23.     Create an srcu_expedite_current() function.
>
> 24.     Test srcu_expedite_current().
>
> 25.     Create an rcu_tasks_trace_expedite_current() function.
>
> 26.     Test rcu_tasks_trace_expedite_current().
>
> 27.     Make DEFINE_SRCU_FAST() available to modules.
>
> 28.     Make SRCU-fast available to heap srcu_struct structures.
>
> 29.     Make grace-period determination use ssp->srcu_reader_flavor.
>
> 30.     Exercise DEFINE_STATIC_SRCU_FAST() and init_srcu_struct_fast().
>
> 31.     Exercise DEFINE_STATIC_SRCU_FAST() and init_srcu_struct_fast().
>
> 32.     Require special srcu_struct define/init for SRCU-fast readers.
>
> 33.     Make SRCU-fast readers enforce use of SRCU-fast definition/init.
>
> 34.     Update for SRCU-fast definitions and initialization.

Maybe it's just me, but the patch set is too fine grained.
These 34 patches could be squashed into a handful for better
review. All these steps: add smp_mb(), make it conditional,
make it more conditional, remove one field,
remove another field is a distraction from actual logic at the end.