Re: [PATCH v2] block: invalidate cached plug timestamp after task switch

[Peter Zijlstra]

On Fri, Jun 12, 2026 at 11:02:58AM +0100, Usama Arif wrote:
> 
> 
> On 12/06/2026 10:45, Peter Zijlstra wrote:
> > On Fri, Jun 12, 2026 at 02:40:42AM -0700, Usama Arif wrote:
> > 
> >> +static __always_inline void blk_plug_invalidate_ts(void)
> >>  {
> >> +	if (unlikely(current->flags & PF_BLOCK_TS)) {
> >> +		struct blk_plug *plug = current->plug;
> >>  
> >> +		if (plug)
> >> +			plug->cur_ktime = 0;
> >> +		current->flags &= ~PF_BLOCK_TS;
> >> +	}
> >>  }
> > 
> > If you can guarantee PF_BLOCK_TS is only ever set when current->plug,
> > this can be reduced further.
> 
> Thanks for the reviews!
> 
> The invariant holds at set time (the only set in blk_time_get_ns() is
> gated by if (!plug)) and through the only legitimate plug clear in
> blk_finish_plug() (which goes through __blk_flush_plug() that clears
> PF_BLOCK_TS first).
> 
> However, copy_process() sets p->plug = NULL for the child but doesn't
> strip PF_BLOCK_TS from the inherited flags.
> 
> I think the if(plug) is a good defensive check, but can also do the below
> if you prefer?

I think that's worth the extra few lines.