linux-kernel - Re: [PATCH v2 04/10] mm/vmalloc: Avoid cond_resched() when blocking is not permitted

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Message-ID: <aMmnkm_E7hDO_yN0@tiehlicka>
Date: Tue, 16 Sep 2025 20:08:18 +0200
From: Michal Hocko <mhocko@...e.com>
To: Uladzislau Rezki <urezki@...il.com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	Baoquan He <bhe@...hat.com>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 04/10] mm/vmalloc: Avoid cond_resched() when blocking
 is not permitted

On Tue 16-09-25 17:28:36, Uladzislau Rezki wrote:
> On Mon, Sep 15, 2025 at 07:11:27PM +0200, Michal Hocko wrote:
> > On Mon 15-09-25 15:40:34, Uladzislau Rezki wrote:
> > > vm_area_alloc_pages() contains the only voluntary reschedule points
> > > along vmalloc() allocation path. They are needed to ensure forward
> > > progress on PREEMPT_NONE kernels under contention for vmap metadata
> > > (e.g. alloc_vmap_area()).
> > > 
> > > However, yielding should only be done if the given GFP flags allow
> > > blocking. This patch avoids calling cond_resched() when allocation
> > > context is non-blocking(GFP_ATOMIC, GFP_NOWAIT).
> > 
> > We do have cond_resched in the page allocator path, right?
> > So unless I am missing something we can safely drope these. I thought we
> > have discused this already.
> > 
> Yes, we discussed this. I did some test with dropped cond_resched() for
> !PREEMPT kernel and i can trigger soft-lockups under really heavy stress
> load.
> 
> I prefer to keep them so far for consistency. I need some time to
> investigate it more. As i noted in commit message, the vmalloc()
> path only has those two resched points. Probably i need to move
> them into another place later.
> 
> As for page-allocator, it is in a slow path which i do not hit in
> my stress-setup.

OK, so the fast path can trigger the soft lockup? If yes please mention
that in the changelog so that we know why this is needed. With that
included feel free to add
Acked-by: Michal Hocko <mhocko@...e.com>

-- 
Michal Hocko
SUSE Labs