Re: [PATCH 7/7] mm/page_alloc: Replace local_lock with normal spinlock

[Mel Gorman <mgorman@techsingularity.net>]

On Thu, Jun 16, 2022 at 07:01:53PM +0200, Vlastimil Babka wrote:
> On 6/13/22 14:56, Mel Gorman wrote:
> > struct per_cpu_pages is no longer strictly local as PCP lists can be
> > drained remotely using a lock for protection. While the use of local_lock
> > works, it goes against the intent of local_lock which is for "pure
> > CPU local concurrency control mechanisms and not suited for inter-CPU
> > concurrency control" (Documentation/locking/locktypes.rst)
> > 
> > local_lock protects against migration between when the percpu pointer is
> > accessed and the pcp->lock acquired. The lock acquisition is a preemption
> > point so in the worst case, a task could migrate to another NUMA node
> > and accidentally allocate remote memory. The main requirement is to pin
> > the task to a CPU that is suitable for PREEMPT_RT and !PREEMPT_RT.
> > 
> > Replace local_lock with helpers that pin a task to a CPU, lookup the
> > per-cpu structure and acquire the embedded lock. It's similar to local_lock
> > without breaking the intent behind the API. It is not a complete API
> > as only the parts needed for PCP-alloc are implemented but in theory,
> > the generic helpers could be promoted to a general API if there was
> > demand for an embedded lock within a per-cpu struct with a guarantee
> > that the per-cpu structure locked matches the running CPU and cannot use
> > get_cpu_var due to RT concerns. PCP requires these semantics to avoid
> > accidentally allocating remote memory.
> > 
> > Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
> 
> ...
> 
> > @@ -3367,30 +3429,17 @@ static int nr_pcp_high(struct per_cpu_pages *pcp, struct zone *zone,
> >  	return min(READ_ONCE(pcp->batch) << 2, high);
> >  }
> >  
> > -/* Returns true if the page was committed to the per-cpu list. */
> > -static bool free_unref_page_commit(struct page *page, int migratetype,
> > -				   unsigned int order, bool locked)
> > +static void free_unref_page_commit(struct per_cpu_pages *pcp, struct zone *zone,
> > +				   struct page *page, int migratetype,
> > +				   unsigned int order)
> 
> Hmm given this drops the "bool locked" and bool return value again, my
> suggestion for patch 5/7 would result in less churn as those woudn't need to
> be introduced?
> 

It would. I considered doing exactly that but I didn't want to drop the
reviewed-bys and tested-bys and the change was significant enough to do
that. As multiple fixes are needed, I'll do that.

> ...
> 
> > @@ -3794,19 +3805,29 @@ static struct page *rmqueue_pcplist(struct zone *preferred_zone,
> >  	struct list_head *list;
> >  	struct page *page;
> >  	unsigned long flags;
> > +	unsigned long __maybe_unused UP_flags;
> >  
> > -	local_lock_irqsave(&pagesets.lock, flags);
> > +	/*
> > +	 * spin_trylock_irqsave is not necessary right now as it'll only be
> > +	 * true when contending with a remote drain. It's in place as a
> > +	 * preparation step before converting pcp locking to spin_trylock
> > +	 * to protect against IRQ reentry.
> > +	 */
> > +	pcp_trylock_prepare(UP_flags);
> > +	pcp = pcp_spin_trylock_irqsave(zone->per_cpu_pageset, flags);
> > +	if (!pcp)
> 
> Besides the missing unpin Andrew fixed, I think also this is missing
> pcp_trylock_finish(UP_flags); ?
> 

Yes.

-- 
Mel Gorman
SUSE Labs