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    sched: Fix softirq time accounting · 75e1056f
    Venkatesh Pallipadi 提交于
    Peter Zijlstra found a bug in the way softirq time is accounted in
    VIRT_CPU_ACCOUNTING on this thread:
    
       http://lkml.indiana.edu/hypermail//linux/kernel/1009.2/01366.html
    
    The problem is, softirq processing uses local_bh_disable internally. There
    is no way, later in the flow, to differentiate between whether softirq is
    being processed or is it just that bh has been disabled. So, a hardirq when bh
    is disabled results in time being wrongly accounted as softirq.
    
    Looking at the code a bit more, the problem exists in !VIRT_CPU_ACCOUNTING
    as well. As account_system_time() in normal tick based accouting also uses
    softirq_count, which will be set even when not in softirq with bh disabled.
    
    Peter also suggested solution of using 2*SOFTIRQ_OFFSET as irq count
    for local_bh_{disable,enable} and using just SOFTIRQ_OFFSET while softirq
    processing. The patch below does that and adds API in_serving_softirq() which
    returns whether we are currently processing softirq or not.
    
    Also changes one of the usages of softirq_count in net/sched/cls_cgroup.c
    to in_serving_softirq.
    
    Looks like many usages of in_softirq really want in_serving_softirq. Those
    changes can be made individually on a case by case basis.
    Signed-off-by: NVenkatesh Pallipadi <venki@google.com>
    Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
    LKML-Reference: <1286237003-12406-2-git-send-email-venki@google.com>
    Signed-off-by: NIngo Molnar <mingo@elte.hu>
    75e1056f
hardirq.h 6.2 KB