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    Subject[PATCH v9 2/7] rcu/segcblist: Add counters to segcblist datastructure
    Date
    Add counting of segment lengths of segmented callback list.

    This will be useful for a number of things such as knowing how big the
    ready-to-execute segment have gotten. The immediate benefit is ability
    to trace how the callbacks in the segmented callback list change.

    Also this patch remove hacks related to using donecbs's ->len field as a
    temporary variable to save the segmented callback list's length. This cannot be
    done anymore and is not needed.

    Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
    Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
    ---
    include/linux/rcu_segcblist.h | 1 +
    kernel/rcu/rcu_segcblist.c | 120 ++++++++++++++++++++++------------
    kernel/rcu/rcu_segcblist.h | 2 -
    3 files changed, 79 insertions(+), 44 deletions(-)

    diff --git a/include/linux/rcu_segcblist.h b/include/linux/rcu_segcblist.h
    index b36afe7b22c9..6c01f09a6456 100644
    --- a/include/linux/rcu_segcblist.h
    +++ b/include/linux/rcu_segcblist.h
    @@ -72,6 +72,7 @@ struct rcu_segcblist {
    #else
    long len;
    #endif
    + long seglen[RCU_CBLIST_NSEGS];
    u8 enabled;
    u8 offloaded;
    };
    diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c
    index bb246d8c6ef1..357c19bbcb00 100644
    --- a/kernel/rcu/rcu_segcblist.c
    +++ b/kernel/rcu/rcu_segcblist.c
    @@ -7,10 +7,11 @@
    * Authors: Paul E. McKenney <paulmck@linux.ibm.com>
    */

    -#include <linux/types.h>
    -#include <linux/kernel.h>
    +#include <linux/cpu.h>
    #include <linux/interrupt.h>
    +#include <linux/kernel.h>
    #include <linux/rcupdate.h>
    +#include <linux/types.h>

    #include "rcu_segcblist.h"

    @@ -88,6 +89,46 @@ static void rcu_segcblist_set_len(struct rcu_segcblist *rsclp, long v)
    #endif
    }

    +/* Get the length of a segment of the rcu_segcblist structure. */
    +static long rcu_segcblist_get_seglen(struct rcu_segcblist *rsclp, int seg)
    +{
    + return READ_ONCE(rsclp->seglen[seg]);
    +}
    +
    +/* Set the length of a segment of the rcu_segcblist structure. */
    +static void rcu_segcblist_set_seglen(struct rcu_segcblist *rsclp, int seg, long v)
    +{
    + WRITE_ONCE(rsclp->seglen[seg], v);
    +}
    +
    +/* Return number of callbacks in a segment of the segmented callback list. */
    +static void rcu_segcblist_add_seglen(struct rcu_segcblist *rsclp, int seg, long v)
    +{
    + WRITE_ONCE(rsclp->seglen[seg], rsclp->seglen[seg] + v);
    +}
    +
    +/* Move from's segment length to to's segment. */
    +static void rcu_segcblist_move_seglen(struct rcu_segcblist *rsclp, int from, int to)
    +{
    + long len;
    +
    + if (from == to)
    + return;
    +
    + len = rcu_segcblist_get_seglen(rsclp, from);
    + if (!len)
    + return;
    +
    + rcu_segcblist_add_seglen(rsclp, to, len);
    + rcu_segcblist_set_seglen(rsclp, from, 0);
    +}
    +
    +/* Increment segment's length. */
    +static void rcu_segcblist_inc_seglen(struct rcu_segcblist *rsclp, int seg)
    +{
    + rcu_segcblist_add_seglen(rsclp, seg, 1);
    +}
    +
    /*
    * Increase the numeric length of an rcu_segcblist structure by the
    * specified amount, which can be negative. This can cause the ->len
    @@ -119,26 +160,6 @@ void rcu_segcblist_inc_len(struct rcu_segcblist *rsclp)
    rcu_segcblist_add_len(rsclp, 1);
    }

    -/*
    - * Exchange the numeric length of the specified rcu_segcblist structure
    - * with the specified value. This can cause the ->len field to disagree
    - * with the actual number of callbacks on the structure. This exchange is
    - * fully ordered with respect to the callers accesses both before and after.
    - */
    -static long rcu_segcblist_xchg_len(struct rcu_segcblist *rsclp, long v)
    -{
    -#ifdef CONFIG_RCU_NOCB_CPU
    - return atomic_long_xchg(&rsclp->len, v);
    -#else
    - long ret = rsclp->len;
    -
    - smp_mb(); /* Up to the caller! */
    - WRITE_ONCE(rsclp->len, v);
    - smp_mb(); /* Up to the caller! */
    - return ret;
    -#endif
    -}
    -
    /*
    * Initialize an rcu_segcblist structure.
    */
    @@ -149,8 +170,10 @@ void rcu_segcblist_init(struct rcu_segcblist *rsclp)
    BUILD_BUG_ON(RCU_NEXT_TAIL + 1 != ARRAY_SIZE(rsclp->gp_seq));
    BUILD_BUG_ON(ARRAY_SIZE(rsclp->tails) != ARRAY_SIZE(rsclp->gp_seq));
    rsclp->head = NULL;
    - for (i = 0; i < RCU_CBLIST_NSEGS; i++)
    + for (i = 0; i < RCU_CBLIST_NSEGS; i++) {
    rsclp->tails[i] = &rsclp->head;
    + rcu_segcblist_set_seglen(rsclp, i, 0);
    + }
    rcu_segcblist_set_len(rsclp, 0);
    rsclp->enabled = 1;
    }
    @@ -246,6 +269,7 @@ void rcu_segcblist_enqueue(struct rcu_segcblist *rsclp,
    {
    rcu_segcblist_inc_len(rsclp);
    smp_mb(); /* Ensure counts are updated before callback is enqueued. */
    + rcu_segcblist_inc_seglen(rsclp, RCU_NEXT_TAIL);
    rhp->next = NULL;
    WRITE_ONCE(*rsclp->tails[RCU_NEXT_TAIL], rhp);
    WRITE_ONCE(rsclp->tails[RCU_NEXT_TAIL], &rhp->next);
    @@ -274,27 +298,13 @@ bool rcu_segcblist_entrain(struct rcu_segcblist *rsclp,
    for (i = RCU_NEXT_TAIL; i > RCU_DONE_TAIL; i--)
    if (rsclp->tails[i] != rsclp->tails[i - 1])
    break;
    + rcu_segcblist_inc_seglen(rsclp, i);
    WRITE_ONCE(*rsclp->tails[i], rhp);
    for (; i <= RCU_NEXT_TAIL; i++)
    WRITE_ONCE(rsclp->tails[i], &rhp->next);
    return true;
    }

    -/*
    - * Extract only the counts from the specified rcu_segcblist structure,
    - * and place them in the specified rcu_cblist structure. This function
    - * supports both callback orphaning and invocation, hence the separation
    - * of counts and callbacks. (Callbacks ready for invocation must be
    - * orphaned and adopted separately from pending callbacks, but counts
    - * apply to all callbacks. Locking must be used to make sure that
    - * both orphaned-callbacks lists are consistent.)
    - */
    -void rcu_segcblist_extract_count(struct rcu_segcblist *rsclp,
    - struct rcu_cblist *rclp)
    -{
    - rclp->len = rcu_segcblist_xchg_len(rsclp, 0);
    -}
    -
    /*
    * Extract only those callbacks ready to be invoked from the specified
    * rcu_segcblist structure and place them in the specified rcu_cblist
    @@ -307,6 +317,7 @@ void rcu_segcblist_extract_done_cbs(struct rcu_segcblist *rsclp,

    if (!rcu_segcblist_ready_cbs(rsclp))
    return; /* Nothing to do. */
    + rclp->len = rcu_segcblist_get_seglen(rsclp, RCU_DONE_TAIL);
    *rclp->tail = rsclp->head;
    WRITE_ONCE(rsclp->head, *rsclp->tails[RCU_DONE_TAIL]);
    WRITE_ONCE(*rsclp->tails[RCU_DONE_TAIL], NULL);
    @@ -314,6 +325,7 @@ void rcu_segcblist_extract_done_cbs(struct rcu_segcblist *rsclp,
    for (i = RCU_CBLIST_NSEGS - 1; i >= RCU_DONE_TAIL; i--)
    if (rsclp->tails[i] == rsclp->tails[RCU_DONE_TAIL])
    WRITE_ONCE(rsclp->tails[i], &rsclp->head);
    + rcu_segcblist_set_seglen(rsclp, RCU_DONE_TAIL, 0);
    }

    /*
    @@ -330,11 +342,16 @@ void rcu_segcblist_extract_pend_cbs(struct rcu_segcblist *rsclp,

    if (!rcu_segcblist_pend_cbs(rsclp))
    return; /* Nothing to do. */
    + rclp->len = rcu_segcblist_get_seglen(rsclp, RCU_WAIT_TAIL) +
    + rcu_segcblist_get_seglen(rsclp, RCU_NEXT_READY_TAIL) +
    + rcu_segcblist_get_seglen(rsclp, RCU_NEXT_TAIL);
    *rclp->tail = *rsclp->tails[RCU_DONE_TAIL];
    rclp->tail = rsclp->tails[RCU_NEXT_TAIL];
    WRITE_ONCE(*rsclp->tails[RCU_DONE_TAIL], NULL);
    - for (i = RCU_DONE_TAIL + 1; i < RCU_CBLIST_NSEGS; i++)
    + for (i = RCU_DONE_TAIL + 1; i < RCU_CBLIST_NSEGS; i++) {
    WRITE_ONCE(rsclp->tails[i], rsclp->tails[RCU_DONE_TAIL]);
    + rcu_segcblist_set_seglen(rsclp, i, 0);
    + }
    }

    /*
    @@ -345,7 +362,6 @@ void rcu_segcblist_insert_count(struct rcu_segcblist *rsclp,
    struct rcu_cblist *rclp)
    {
    rcu_segcblist_add_len(rsclp, rclp->len);
    - rclp->len = 0;
    }

    /*
    @@ -359,6 +375,7 @@ void rcu_segcblist_insert_done_cbs(struct rcu_segcblist *rsclp,

    if (!rclp->head)
    return; /* No callbacks to move. */
    + rcu_segcblist_add_seglen(rsclp, RCU_DONE_TAIL, rclp->len);
    *rclp->tail = rsclp->head;
    WRITE_ONCE(rsclp->head, rclp->head);
    for (i = RCU_DONE_TAIL; i < RCU_CBLIST_NSEGS; i++)
    @@ -379,6 +396,8 @@ void rcu_segcblist_insert_pend_cbs(struct rcu_segcblist *rsclp,
    {
    if (!rclp->head)
    return; /* Nothing to do. */
    +
    + rcu_segcblist_add_seglen(rsclp, RCU_NEXT_TAIL, rclp->len);
    WRITE_ONCE(*rsclp->tails[RCU_NEXT_TAIL], rclp->head);
    WRITE_ONCE(rsclp->tails[RCU_NEXT_TAIL], rclp->tail);
    }
    @@ -403,6 +422,7 @@ void rcu_segcblist_advance(struct rcu_segcblist *rsclp, unsigned long seq)
    if (ULONG_CMP_LT(seq, rsclp->gp_seq[i]))
    break;
    WRITE_ONCE(rsclp->tails[RCU_DONE_TAIL], rsclp->tails[i]);
    + rcu_segcblist_move_seglen(rsclp, i, RCU_DONE_TAIL);
    }

    /* If no callbacks moved, nothing more need be done. */
    @@ -423,6 +443,7 @@ void rcu_segcblist_advance(struct rcu_segcblist *rsclp, unsigned long seq)
    if (rsclp->tails[j] == rsclp->tails[RCU_NEXT_TAIL])
    break; /* No more callbacks. */
    WRITE_ONCE(rsclp->tails[j], rsclp->tails[i]);
    + rcu_segcblist_move_seglen(rsclp, i, j);
    rsclp->gp_seq[j] = rsclp->gp_seq[i];
    }
    }
    @@ -444,7 +465,7 @@ void rcu_segcblist_advance(struct rcu_segcblist *rsclp, unsigned long seq)
    */
    bool rcu_segcblist_accelerate(struct rcu_segcblist *rsclp, unsigned long seq)
    {
    - int i;
    + int i, j;

    WARN_ON_ONCE(!rcu_segcblist_is_enabled(rsclp));
    if (rcu_segcblist_restempty(rsclp, RCU_DONE_TAIL))
    @@ -487,6 +508,10 @@ bool rcu_segcblist_accelerate(struct rcu_segcblist *rsclp, unsigned long seq)
    if (rcu_segcblist_restempty(rsclp, i) || ++i >= RCU_NEXT_TAIL)
    return false;

    + /* Accounting: everything below i is about to get merged into i. */
    + for (j = i + 1; j <= RCU_NEXT_TAIL; j++)
    + rcu_segcblist_move_seglen(rsclp, j, i);
    +
    /*
    * Merge all later callbacks, including newly arrived callbacks,
    * into the segment located by the for-loop above. Assign "seq"
    @@ -514,13 +539,24 @@ void rcu_segcblist_merge(struct rcu_segcblist *dst_rsclp,
    struct rcu_cblist donecbs;
    struct rcu_cblist pendcbs;

    + lockdep_assert_cpus_held();
    +
    rcu_cblist_init(&donecbs);
    rcu_cblist_init(&pendcbs);
    - rcu_segcblist_extract_count(src_rsclp, &donecbs);
    +
    rcu_segcblist_extract_done_cbs(src_rsclp, &donecbs);
    rcu_segcblist_extract_pend_cbs(src_rsclp, &pendcbs);
    +
    + /*
    + * No need smp_mb() before setting length to 0, because CPU hotplug
    + * lock excludes rcu_barrier.
    + */
    + rcu_segcblist_set_len(src_rsclp, 0);
    +
    rcu_segcblist_insert_count(dst_rsclp, &donecbs);
    + rcu_segcblist_insert_count(dst_rsclp, &pendcbs);
    rcu_segcblist_insert_done_cbs(dst_rsclp, &donecbs);
    rcu_segcblist_insert_pend_cbs(dst_rsclp, &pendcbs);
    +
    rcu_segcblist_init(src_rsclp);
    }
    diff --git a/kernel/rcu/rcu_segcblist.h b/kernel/rcu/rcu_segcblist.h
    index 1d2d61406463..cd35c9faaf51 100644
    --- a/kernel/rcu/rcu_segcblist.h
    +++ b/kernel/rcu/rcu_segcblist.h
    @@ -89,8 +89,6 @@ void rcu_segcblist_enqueue(struct rcu_segcblist *rsclp,
    struct rcu_head *rhp);
    bool rcu_segcblist_entrain(struct rcu_segcblist *rsclp,
    struct rcu_head *rhp);
    -void rcu_segcblist_extract_count(struct rcu_segcblist *rsclp,
    - struct rcu_cblist *rclp);
    void rcu_segcblist_extract_done_cbs(struct rcu_segcblist *rsclp,
    struct rcu_cblist *rclp);
    void rcu_segcblist_extract_pend_cbs(struct rcu_segcblist *rsclp,
    --
    2.29.1.341.ge80a0c044ae-goog
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