Re: [PATCH v5 5/5] rqspinlock: Use smp_cond_load_acquire_timeout()
From: Catalin Marinas <catalin.marinas@arm.com>
Date: 2025-09-12 10:14:38
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On Thu, Sep 11, 2025 at 02:58:22PM -0700, Ankur Arora wrote:
Kumar Kartikeya Dwivedi [off-list ref] writes:quoted
On Thu, 11 Sept 2025 at 16:32, Catalin Marinas [off-list ref] wrote:quoted
On Wed, Sep 10, 2025 at 08:46:55PM -0700, Ankur Arora wrote:quoted
Switch out the conditional load inerfaces used by rqspinlock to smp_cond_read_acquire_timeout(). This interface handles the timeout check explicitly and does any necessary amortization, so use check_timeout() directly.It's worth mentioning that the default smp_cond_load_acquire_timeout() implementation (without hardware support) only spins 200 times instead of 16K times in the rqspinlock code. That's probably fine but it would be good to have confirmation from Kumar or Alexei.This looks good, but I would still redefine the spin count from 200 to 16k for rqspinlock.c, especially because we need to keep RES_CHECK_TIMEOUT around which still uses 16k spins to amortize check_timeout.By my count that amounts to ~100us per check_timeout() on x86 systems I've tested with cpu_relax(). Which seems quite reasonable. 16k also seems safer on CPUs where cpu_relax() is basically a NOP.
Does this spin count work for poll_idle()? I don't remember where the 200 value came from. -- Catalin