• Steven Rostedt's avatar
    x86: enable preemption in delay · 5c1ea082
    Steven Rostedt authored
    The RT team has been searching for a nasty latency. This latency shows
    up out of the blue and has been seen to be as big as 5ms!
    
    Using ftrace I found the cause of the latency.
    
       pcscd-2995  3dNh1 52360300us : irq_exit (smp_apic_timer_interrupt)
       pcscd-2995  3dN.2 52360301us : idle_cpu (irq_exit)
       pcscd-2995  3dN.2 52360301us : rcu_irq_exit (irq_exit)
       pcscd-2995  3dN.1 52360771us : smp_apic_timer_interrupt (apic_timer_interrupt
    )
       pcscd-2995  3dN.1 52360771us : exit_idle (smp_apic_timer_interrupt)
    
    Here's an example of a 400 us latency. pcscd took a timer interrupt and
    returned with "need resched" enabled, but did not reschedule until after
    the next interrupt came in at 52360771us 400us later!
    
    At first I thought we somehow missed a preemption check in entry.S. But
    I also noticed that this always seemed to happen during a __delay call.
    
       pcscd-2995  3dN.2 52360836us : rcu_irq_exit (irq_exit)
       pcscd-2995  3.N.. 52361265us : preempt_schedule (__delay)
    
    Looking at the x86 delay, I found my problem.
    
    In git commit 35d5d08a, Andrew Morton
    placed preempt_disable around the entire delay due to TSC's not working
    nicely on SMP.  Unfortunately for those that care about latencies this
    is devastating! Especially when we have callers to mdelay(8).
    
    Here I enable preemption during the loop and account for anytime the task
    migrates to a new CPU. The delay asked for may be extended a bit by
    the migration, but delay only guarantees that it will delay for that minimum
    time. Delaying longer should not be an issue.
    
    [
      Thanks to Thomas Gleixner for spotting that cpu wasn't updated,
        and to place the rep_nop between preempt_enabled/disable.
    ]
    Signed-off-by: default avatarSteven Rostedt <srostedt@redhat.com>
    Cc: akpm@osdl.org
    Cc: Clark Williams <clark.williams@gmail.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: "Luis Claudio R. Goncalves" <lclaudio@uudg.org>
    Cc: Gregory Haskins <ghaskins@novell.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Andi Kleen <andi-suse@firstfloor.org>
    Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
    5c1ea082
delay_64.c 1.85 KB