• Joel Fernandes (Google)'s avatar
    tracepoint: Make rcuidle tracepoint callers use SRCU · e6753f23
    Joel Fernandes (Google) authored
    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.orgCleaned-up-by: default avatarPeter Zijlstra <peterz@infradead.org>
    Acked-by: default avatarPeter Zijlstra <peterz@infradead.org>
    Reviewed-by: default avatarMathieu Desnoyers <mathieu.desnoyers@efficios.com>
    Signed-off-by: default avatarJoel Fernandes (Google) <joel@joelfernandes.org>
    [ Simplified WARN_ON_ONCE() ]
    Signed-off-by: default avatarSteven Rostedt (VMware) <rostedt@goodmis.org>
    e6753f23
tracepoint.h 16.8 KB