rcu: Reduce synchronize_rcu() latency by reporting GP kthread's CPU QS early

rcu: Reduce synchronize_rcu() latency by reporting GP kthread's CPU QS early

The RCU grace period mechanism uses a two-phase FQS (Force Quiescent
State) design where the first FQS saves dyntick-idle snapshots and
the second FQS compares them. This results in long and unnecessary latency
for synchronize_rcu() on idle systems (two FQS waits of ~3ms each with
1000HZ) whenever one FQS wait sufficed.

Some investigations showed that the GP kthread's CPU is the holdout CPU
a lot of times after the first FQS as - it cannot be detected as "idle"
because it's actively running the FQS scan in the GP kthread.

Therefore, at the end of rcu_gp_init(), immediately report a quiescent
state for the GP kthread's CPU using rcu_qs() + rcu_report_qs_rdp(). The
GP kthread cannot be in an RCU read-side critical section while running
GP initialization, so this is safe and results in significant latency
improvements.

The following tests were performed:

(1) synchronize_rcu() benchmarking

    100 synchronize_rcu() calls with 32 CPUs, 10 runs each (default fqs
    jiffies settings):

    Baseline (without fix):
    | Run | Mean      | Min      | Max       |
    |-----|-----------|----------|-----------|
    | 1   | 10.088 ms | 9.989 ms | 18.848 ms |
    | 2   | 10.064 ms | 9.982 ms | 16.470 ms |
    | 3   | 10.051 ms | 9.988 ms | 15.113 ms |
    | 4   | 10.125 ms | 9.929 ms | 22.411 ms |
    | 5   |  8.695 ms | 5.996 ms | 15.471 ms |
    | 6   | 10.157 ms | 9.977 ms | 25.723 ms |
    | 7   | 10.102 ms | 9.990 ms | 20.224 ms |
    | 8   |  8.050 ms | 5.985 ms | 10.007 ms |
    | 9   | 10.059 ms | 9.978 ms | 15.934 ms |
    | 10  | 10.077 ms | 9.984 ms | 17.703 ms |

    With fix:
    | Run | Mean     | Min      | Max       |
    |-----|----------|----------|-----------|
    | 1   | 6.027 ms | 5.915 ms |  8.589 ms |
    | 2   | 6.032 ms | 5.984 ms |  9.241 ms |
    | 3   | 6.010 ms | 5.986 ms |  7.004 ms |
    | 4   | 6.076 ms | 5.993 ms | 10.001 ms |
    | 5   | 6.084 ms | 5.893 ms | 10.250 ms |
    | 6   | 6.034 ms | 5.908 ms |  9.456 ms |
    | 7   | 6.051 ms | 5.993 ms | 10.000 ms |
    | 8   | 6.057 ms | 5.941 ms | 10.001 ms |
    | 9   | 6.016 ms | 5.927 ms |  7.540 ms |
    | 10  | 6.036 ms | 5.993 ms |  9.579 ms |

    Summary:
    - Mean latency: 9.75 ms -> 6.04 ms (38% improvement)
    - Max latency:  25.72 ms -> 10.25 ms (60% improvement)

(2) Bridge setup/teardown latency (Uladzislau Rezki)

    x86_64 with 64 CPUs, 100 iterations of bridge add/configure/delete:

                                   real time
    1 - default:                   24.221s
    2 - this patch:                20.754s  (14% faster)
    3 - this patch + wake_from_gp: 15.895s  (34% faster)
    4 - wake_from_gp only:         18.947s  (22% faster)

    Per-synchronize_rcu() latency (in usec):
                  1         2         3       4
    median: 37249.5   31540.5   15765   22480
    min:    7881      7918      9803    7857
    max:    63651     55639     31861   32040

    This patch combined with rcu_normal_wake_from_gp reduces bridge
    setup/teardown time from 24 seconds to 16 seconds.

(3) CPU overhead verification (Uladzislau Rezki)

    System CPU time across 5 runs showed no measurable increase:
      default:     1.698s - 1.937s
      this patch:  1.667s - 1.930s
    Conclusion: variations are within noise, no CPU overhead regression.

(4) rcutorture

    Tested TREE and SRCU configurations - no regressions.

Reviewed-by: "Paul E. McKenney" <paulmck@kernel.org>
Tested-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Tested-by: Paul E. McKenney <paulmck@kernel.org>
Tested-by: Samir M <samir@linux.ibm.com>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>