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    Subject[for-next][PATCH 15/34] tracepoint: Make rcuidle tracepoint callers use SRCU
    From: "Joel Fernandes (Google)" <joel@joelfernandes.org>

    In recent tests with IRQ on/off tracepoints, a large performance
    overhead ~10% is noticed when running hackbench. This is root caused to
    calls to rcu_irq_enter_irqson and rcu_irq_exit_irqson from the
    tracepoint code. Following a long discussion on the list [1] about this,
    we concluded that srcu is a better alternative for use during rcu idle.
    Although it does involve extra barriers, its lighter than the sched-rcu
    version which has to do additional RCU calls to notify RCU idle about
    entry into RCU sections.

    In this patch, we change the underlying implementation of the
    trace_*_rcuidle API to use SRCU. This has shown to improve performance
    alot for the high frequency irq enable/disable tracepoints.

    Test: Tested idle and preempt/irq tracepoints.

    Here are some performance numbers:

    With a run of the following 30 times on a single core x86 Qemu instance
    with 1GB memory:
    hackbench -g 4 -f 2 -l 3000

    Completion times in seconds. CONFIG_PROVE_LOCKING=y.

    No patches (without this series)
    Mean: 3.048
    Median: 3.025
    Std Dev: 0.064

    With Lockdep using irq tracepoints with RCU implementation:
    Mean: 3.451 (-11.66 %)
    Median: 3.447 (-12.22%)
    Std Dev: 0.049

    With Lockdep using irq tracepoints with SRCU implementation (this series):
    Mean: 3.020 (I would consider the improvement against the "without
    this series" case as just noise).
    Median: 3.013
    Std Dev: 0.033

    [1] https://patchwork.kernel.org/patch/10344297/

    [remove rcu_read_lock_sched_notrace as its the equivalent of
    preempt_disable_notrace and is unnecessary to call in tracepoint code]
    Link: http://lkml.kernel.org/r/20180730222423.196630-3-joel@joelfernandes.org

    Cleaned-up-by: Peter Zijlstra <peterz@infradead.org>
    Acked-by: Peter Zijlstra <peterz@infradead.org>
    Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
    Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
    [ Simplified WARN_ON_ONCE() ]
    Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
    ---
    include/linux/tracepoint.h | 40 ++++++++++++++++++++++++++++++--------
    kernel/tracepoint.c | 16 ++++++++++++++-
    2 files changed, 47 insertions(+), 9 deletions(-)

    diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
    index 19a690b559ca..d9a084c72541 100644
    --- a/include/linux/tracepoint.h
    +++ b/include/linux/tracepoint.h
    @@ -15,6 +15,7 @@
    */

    #include <linux/smp.h>
    +#include <linux/srcu.h>
    #include <linux/errno.h>
    #include <linux/types.h>
    #include <linux/cpumask.h>
    @@ -33,6 +34,8 @@ struct trace_eval_map {

    #define TRACEPOINT_DEFAULT_PRIO 10

    +extern struct srcu_struct tracepoint_srcu;
    +
    extern int
    tracepoint_probe_register(struct tracepoint *tp, void *probe, void *data);
    extern int
    @@ -75,10 +78,16 @@ int unregister_tracepoint_module_notifier(struct notifier_block *nb)
    * probe unregistration and the end of module exit to make sure there is no
    * caller executing a probe when it is freed.
    */
    +#ifdef CONFIG_TRACEPOINTS
    static inline void tracepoint_synchronize_unregister(void)
    {
    + synchronize_srcu(&tracepoint_srcu);
    synchronize_sched();
    }
    +#else
    +static inline void tracepoint_synchronize_unregister(void)
    +{ }
    +#endif

    #ifdef CONFIG_HAVE_SYSCALL_TRACEPOINTS
    extern int syscall_regfunc(void);
    @@ -129,18 +138,31 @@ extern void syscall_unregfunc(void);
    * as "(void *, void)". The DECLARE_TRACE_NOARGS() will pass in just
    * "void *data", where as the DECLARE_TRACE() will pass in "void *data, proto".
    */
    -#define __DO_TRACE(tp, proto, args, cond, rcucheck) \
    +#define __DO_TRACE(tp, proto, args, cond, rcuidle) \
    do { \
    struct tracepoint_func *it_func_ptr; \
    void *it_func; \
    void *__data; \
    + int __maybe_unused idx = 0; \
    \
    if (!(cond)) \
    return; \
    - if (rcucheck) \
    - rcu_irq_enter_irqson(); \
    - rcu_read_lock_sched_notrace(); \
    - it_func_ptr = rcu_dereference_sched((tp)->funcs); \
    + \
    + /* srcu can't be used from NMI */ \
    + WARN_ON_ONCE(rcuidle && in_nmi()); \
    + \
    + /* keep srcu and sched-rcu usage consistent */ \
    + preempt_disable_notrace(); \
    + \
    + /* \
    + * For rcuidle callers, use srcu since sched-rcu \
    + * doesn't work from the idle path. \
    + */ \
    + if (rcuidle) \
    + idx = srcu_read_lock_notrace(&tracepoint_srcu); \
    + \
    + it_func_ptr = rcu_dereference_raw((tp)->funcs); \
    + \
    if (it_func_ptr) { \
    do { \
    it_func = (it_func_ptr)->func; \
    @@ -148,9 +170,11 @@ extern void syscall_unregfunc(void);
    ((void(*)(proto))(it_func))(args); \
    } while ((++it_func_ptr)->func); \
    } \
    - rcu_read_unlock_sched_notrace(); \
    - if (rcucheck) \
    - rcu_irq_exit_irqson(); \
    + \
    + if (rcuidle) \
    + srcu_read_unlock_notrace(&tracepoint_srcu, idx);\
    + \
    + preempt_enable_notrace(); \
    } while (0)

    #ifndef MODULE
    diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
    index 6dc6356c3327..955148d91b74 100644
    --- a/kernel/tracepoint.c
    +++ b/kernel/tracepoint.c
    @@ -31,6 +31,9 @@
    extern struct tracepoint * const __start___tracepoints_ptrs[];
    extern struct tracepoint * const __stop___tracepoints_ptrs[];

    +DEFINE_SRCU(tracepoint_srcu);
    +EXPORT_SYMBOL_GPL(tracepoint_srcu);
    +
    /* Set to 1 to enable tracepoint debug output */
    static const int tracepoint_debug;

    @@ -67,16 +70,27 @@ static inline void *allocate_probes(int count)
    return p == NULL ? NULL : p->probes;
    }

    -static void rcu_free_old_probes(struct rcu_head *head)
    +static void srcu_free_old_probes(struct rcu_head *head)
    {
    kfree(container_of(head, struct tp_probes, rcu));
    }

    +static void rcu_free_old_probes(struct rcu_head *head)
    +{
    + call_srcu(&tracepoint_srcu, head, srcu_free_old_probes);
    +}
    +
    static inline void release_probes(struct tracepoint_func *old)
    {
    if (old) {
    struct tp_probes *tp_probes = container_of(old,
    struct tp_probes, probes[0]);
    + /*
    + * Tracepoint probes are protected by both sched RCU and SRCU,
    + * by calling the SRCU callback in the sched RCU callback we
    + * cover both cases. So let us chain the SRCU and sched RCU
    + * callbacks to wait for both grace periods.
    + */
    call_rcu_sched(&tp_probes->rcu, rcu_free_old_probes);
    }
    }
    --
    2.18.0

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