Re: [PATCH] sched: Revert need_resched() to look at TIF_NEED_RESCHED

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

Peter Zijlstra <peterz@infradead.org> writes:

> Subject: sched: Revert need_resched() to look at TIF_NEED_RESCHED
> From: Peter Zijlstra <peterz@infradead.org>
> Date: Fri Sep 27 17:20:30 CEST 2013
>
> Yuanhan reported a serious throughput regression in his pigz
> benchmark. Using the ftrace patch I found that several idle paths
> need more TLC before we can switch the generic need_resched() over to
> preempt_need_resched.
>
> The preemption paths benefit most from preempt_need_resched and do
> indeed use it; all other need_resched() users don't really care that
> much so reverting need_resched() back to tif_need_resched() is the
> simple and safe solution.
>
> Reported-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
> Signed-off-by: Peter Zijlstra <peterz@infradead.org>

I guess we still need to fix should_resched() I am hitting the below
with upstream


BUG: soft lockup - CPU#0 stuck for 23s! [qemu-system-ppc:4394]
Modules linked in:
CPU: 0 PID: 4394 Comm: qemu-system-ppc Not tainted 3.13.0-rc3+ #98
task: c0000001d0788400 ti: c0000001dca00000 task.ti: c0000001dca00000
NIP: c00000000082dd80 LR: c000000000081ae0 CTR: c000000000062ba0
REGS: c0000001dca02f70 TRAP: 0901   Not tainted  (3.13.0-rc3+)
MSR: 8000000000009032 <SF,EE,ME,IR,DR,RI>  CR: 24822024  XER: 00000000
CFAR: c000000000081adc SOFTE: 1
GPR00: c000000000081ae0 c0000001dca031f0 c000000000d67ab0 0000000000000001
GPR04: 0000000071000002 0000000000000001 00000189a0d786b7 00000000018c0000
GPR08: 0000000000000001 0000000000000000 0000000000000000 c000000000da0000
GPR12: 0000000000000c00 c00000000fef0000
NIP [c00000000082dd80] ._cond_resched+0x0/0x40
LR [c000000000081ae0] .kvmppc_prepare_to_enter+0x2a0/0x2e0
Call Trace:
[c0000001dca031f0] [c000000000081ae0] .kvmppc_prepare_to_enter+0x2a0/0x2e0 (unreliable)
[c0000001dca03290] [c00000000008f2cc] .kvmppc_handle_exit_pr+0xec/0xa40
[c0000001dca03340] [c0000000000918c4] kvm_start_lightweight+0xac/0xbc
[c0000001dca03510] [c00000000008efe0] .kvmppc_vcpu_run_pr+0x130/0x2a0
[c0000001dca039e0] [c0000000000855bc] .kvmppc_vcpu_run+0x2c/0x40
[c0000001dca03a50] [c000000000082c94] .kvm_arch_vcpu_ioctl_run+0x54/0x1b0
[c0000001dca03ae0] [c00000000007d5f8] .kvm_vcpu_ioctl+0x478/0x740
[c0000001dca03ca0] [c000000000218864] .do_vfs_ioctl+0x4a4/0x760
[c0000001dca03d80] [c000000000218b78] .SyS_ioctl+0x58/0xb0
[c0000001dca03e30] [c000000000009e58] syscall_exit+0x0/0x98
Instruction dump:
e92d0260 e94911c0 812a0004 5529f07e 5529103e 912a0004 38210080 e8010010
ebc1fff0 ebe1fff8 7c0803a6 4e800020 <7c0802a6> 38600000 f8010010 f821ff91 


The revert which Alexander tried had those

http://mid.gmane.org/20131128132641.GP10022@twins.programming.kicks-ass.net


> ---
>  arch/x86/include/asm/preempt.h |    8 --------
>  include/asm-generic/preempt.h  |    8 --------
>  include/linux/sched.h          |    5 +++++
>  3 files changed, 5 insertions(+), 16 deletions(-)
>
> --- a/arch/x86/include/asm/preempt.h
> +++ b/arch/x86/include/asm/preempt.h
> @@ -80,14 +80,6 @@ static __always_inline bool __preempt_co
>  }
>  
>  /*
> - * Returns true when we need to resched -- even if we can not.
> - */
> -static __always_inline bool need_resched(void)
> -{
> -	return unlikely(test_preempt_need_resched());
> -}
> -
> -/*
>   * Returns true when we need to resched and can (barring IRQ state).
>   */
>  static __always_inline bool should_resched(void)
> --- a/include/asm-generic/preempt.h
> +++ b/include/asm-generic/preempt.h
> @@ -85,14 +85,6 @@ static __always_inline bool __preempt_co
>  }
>  
>  /*
> - * Returns true when we need to resched -- even if we can not.
> - */
> -static __always_inline bool need_resched(void)
> -{
> -	return unlikely(test_preempt_need_resched());
> -}
> -
> -/*
>   * Returns true when we need to resched and can (barring IRQ state).
>   */
>  static __always_inline bool should_resched(void)
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -2577,6 +2577,11 @@ static inline bool __must_check current_
>  }
>  #endif
>  
> +static __always_inline bool need_resched(void)
> +{
> +	return unlikely(tif_need_resched());
> +}
> +
>  /*
>   * Thread group CPU time accounting.
>   */

-aneesh