Re: [PATCH v1 00/11] RCU: Enable callbacks to benefit from expedited grace periods

On Wed, Jun 24, 2026 at 06:23:42AM -0700, Puranjay Mohan wrote:

This series lets call_rcu() callbacks be reclaimed as soon as either a
normal or an expedited grace period that covers them has elapsed, rather
than always waiting for a normal grace period.

Motivation
==========
Today there is an asymmetry: synchronize_rcu_expedited() callers get fast
reclaim, but call_rcu() callers never benefit from those same expedited
grace periods, even though an expedited GP proves exactly the same thing
as a normal one -- all pre-existing readers are done.  When expedited GPs
are running on the system (driven by other subsystems), call_rcu()
callbacks that could already be freed instead sit in RCU_WAIT_TAIL until
the next normal GP.  This series treats a grace period as a grace period
regardless of how it was driven, so memory is reclaimed sooner.

Design
======
Callback segments now record both the normal and expedited grace-period
sequence in struct rcu_gp_seq, and rcu_segcblist_advance() releases a
segment as soon as poll_state_synchronize_rcu_full() reports that either
has completed.  Three notification paths are taught about expedited
completion so the advance actually happens: the NOCB rcuog kthreads,
the rcu_pending() tick gate, and rcu_core().

Changelog:
RFC: https://lore.kernel.org/all/20260417231203.785172-1-puranjay@kernel.org/ (local)
Changes in v1:
 - New prep patch 1 renames struct rcu_gp_oldstate to struct rcu_gp_seq
   and its fields rgos_norm/rgos_exp to norm/exp tree-wide (Frederic).
 - The rcu_segcblist segment field stays named gp_seq; only its type
   changes (Frederic).
 - Patch 8 (NOCB wake) is reworked.  v1 woke the wrong waitqueue
   (rdp_gp->nocb_gp_wq via wake_nocb_gp() rather than the leaf
   rnp->nocb_gp_wq[] that an rcuog kthread waiting for a GP sleeps on),
   and the wait condition only checked the normal ->gp_seq.  The rcuog
   grace-period wait now tracks a struct rcu_gp_seq and is released via
   poll_state_synchronize_rcu_full(); rcu_exp_wait_wake() wakes the leaf
   node through the new rcu_nocb_exp_cleanup() (Frederic).
 - rcu_pending() uses a new memory-ordering-free
   poll_state_synchronize_rcu_full_unordered() to avoid memory barriers
   on every tick, leaving the ordering duty to rcu_core() (Frederic).

Still open: Frederic asked whether the first smp_mb() in
poll_state_synchronize_rcu_full() is needed on the callback-advance path
(patch 6).  That path still uses the fully ordered helper; only
rcu_pending() was switched to the unordered variant.  Happy to revisit.

Puranjay Mohan (11):
  rcu: Rename struct rcu_gp_oldstate to rcu_gp_seq
  rcu/segcblist: Add SRCU and Tasks RCU wrapper functions
  rcu/segcblist: Factor out rcu_segcblist_advance_compact() helper
  rcu/segcblist: Track segment grace periods with struct rcu_gp_seq
  rcu: Add RCU_GET_STATE_NOT_TRACKED for subsystems without expedited
    GPs
  rcu: Enable RCU callbacks to benefit from expedited grace periods
  rcu: Update comments for gp_seq and expedited GP tracking
  rcu: Wake NOCB rcuog kthreads on expedited grace period completion
  rcu: Detect expedited grace period completion in rcu_pending()
  rcu: Advance callbacks for expedited GP completion in rcu_core()
  rcuscale: Add concurrent expedited GP threads for callback scaling
    tests

I do see the occasional failure when running TREE01, as in a failure or
four every couple hundred hours of testing.  This is the only rcutorture
scenario that exercises callback (de)offloading, which might (or might
not) be a clue.

The console log of a failing run is attached, or you can find it here:

https://drive.google.com/file/d/1EH2P4SDy2a-SJndkN3GbCTGAXxI2xevm/view?usp=sharing

This is a "??? Writer stall state" failure, where rcutorture never learns
of grace-period completion.  Possibly because no grace period completed.
But there are no RCU CPU stall warnings.  In addition, these two lines

[  631.032657] ??? Writer stall state RTWS_EXP_SYNC(4) g152813 f0x0 ->state D cpu 1
. . .
[ 3610.863114] ??? Writer stall state RTWS_EXP_SYNC(4) g736160 f0x0 ->state D cpu 1

show that the grace-period sequence number advanced from 152813 to
736160 in about 50 minutes.  So grace periods really are completing,
lots of them.

This rcutorture scenario has rcu_nocbs CPUs, and CPU 5 is in trouble:

[  631.066109] rcu:    CB 5^4->4 KblSW F21044 L21044 C116 .W120(148380/168068).N295B101 q516 S CPU 16
. . .
[ 3610.869914] rcu:    CB 5^4->4 KblSW F3000848 L3000848 C116 .W120(148380/168068).N295B101 q516 S CPU 16

There are 120 callbacks on the WAIT segment, which are waiting on
grace-period sequence number 148380, which was long gone even back at
631.032657 seconds in (g152813).  There are 295 more callbacks in the NEXT
segment, which long ago should have been assigned a grace period and moved
into the currently empty NEXT_READY segment (which is the "." right before
the "N295").  There are yet another 101 callbacks on the bypass list.

This suggests that the rcuog kthread isn't being awakened when it
should be.  And that something is failing to advance callbacks out of
NEXT in a timely fashion.

Thoughts?

							Thanx, Paul