From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 434A830CDBC for ; Tue, 18 Aug 2026 18:21:42 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787077303; cv=none; b=lAF2oTP8x3RO/+V10P/RCaWaDfVpJ+Us4vQZEpoRUbP7OSUx0N0DhoOEcKZXqPou2EqSX1P6CxQsP3KLxS9tIL8VFMjHosux7sVaygIsk4MLR4fFppEl7OZgWXMnabudlMzxYwwqDCCVlCynB2hDEvUri8F8AkiSbS7NbeZkLBA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787077303; c=relaxed/simple; bh=xPlLA5TkMaSfPwv6gMDv88wwyww1Ayh1naXWW4w9mVg=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=tdPy2n4PNHmnTXoWeH7Lg5XlP/LDlaprjuZmHTs1V7CsPkDGVDB5JmQiiaoKmSIm1BDYGiXjoBgd0NWUcDVgG7pCrGRKbE+Muffq7MYwAmMPc1CJscW8C4DtyLbdgzJ7/FC7rPW+52L3ccKVH3e2UQzM35kmjhXfwyx+TMEu13A= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Ha70fL+a; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="Ha70fL+a" Received: by smtp.kernel.org (Postfix) with ESMTPSA id EC9B61F000E9; Tue, 18 Aug 2026 18:21:39 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1787077302; bh=oTaxowGYxPAPGFyxd1NFA2Af0dHR5XEqV4XaIiJGh1k=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=Ha70fL+aGh3fQT2cA0gDU+rSONQ0xfV4rN9SR4DUJkI3H6GrHl41CDWWp+uT1UrLb dxa1AGDS5pE0KOkya2w8jTciQewaLuKhhoKVvXPTgCMF0m94/z/Tan5uAJ2CsTvUzd 2recbwMoxdDzn2CC8nsw3siQXHUXggEzQ4trU34NSrDJ1NANqUBRR3Yu9niAlBRqHX hkL6sqeBGDCMhNFL8XwIZCnWQh/EYRueccbzeY2i1fytTpmWQUBJN33FU6ep0nvaGQ q6IRduP/fdiAt50DgJlrsk8pZViQlyMyUfExEVMm583/dI4uDYnwSa0SyCRy8UI6nO 1S9zStDyNiggw== Message-ID: <2a523945-825c-45a5-a680-37e941e092d3@kernel.org> Date: Tue, 18 Aug 2026 20:21:37 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] hugetlb: add cond_resched() to __unmap_hugepage_range() To: Andrew Morton Cc: Leon Hwang , linux-mm@kvack.org, Muchun Song , Oscar Salvador , linux-kernel@vger.kernel.org, Lance Yang References: <20260818135029.93288-1-leon.hwang@linux.dev> <3dadebb3-7e2a-459c-a5e8-b375faf89938@kernel.org> <20260818111756.6b0e347db3170bc22cc3c5af@linux-foundation.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 8/18/26 20:17, Andrew Morton wrote: > On Tue, 18 Aug 2026 16:23:29 +0200 "David Hildenbrand (Arm)" wrote: > >> On 8/18/26 15:50, Leon Hwang wrote: >>> Packet receive timeouts were traced to sparse HugeTLB unmapping in >>> production. A task unmapping a sparse 2.5 TiB HugeTLB mapping could >>> remain in kernel context for over 40 ms without reaching a scheduling >>> point while walking empty huge PTEs. Although hard IRQs could still be >>> handled, the per-CPU ksoftirqd thread and other runnable tasks could not >>> run during that interval, delaying NET_RX softirq work queued to >>> ksoftirqd. >>> >>> Add cond_resched() at the beginning of the hugepage loop so ksoftirqd >>> and other runnable tasks can run between iterations. Testing with >>> PREEMPT_NONE showed that the maximum interval between scheduling points >>> fell from over 40 ms to below 2.5 ms. Total time spent in >>> __unmap_hugepage_range() remained about 36 ms. >>> >>> Reported-by: Lance Yang >>> Tested-by: Lance Yang > > Is there a Link: to Lance's report? > >>> Signed-off-by: Leon Hwang >>> --- >>> mm/hugetlb.c | 2 ++ >>> 1 file changed, 2 insertions(+) >>> >>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c >>> index dded1768193a..0a91aac2369f 100644 >>> --- a/mm/hugetlb.c >>> +++ b/mm/hugetlb.c >>> @@ -5233,6 +5233,8 @@ void __unmap_hugepage_range(struct mmu_gather *tlb, struct vm_area_struct *vma, >>> last_addr_mask = hugetlb_mask_last_page(h); >>> address = start; >>> for (; address < end; address += sz) { >>> + cond_resched(); >>> + >>> ptep = hugetlb_walk(vma, address, sz); >>> if (!ptep) { >>> address |= last_addr_mask; >> >> As Michal just put it: >> >> "PREEMPT_NONE is effectivelly dead and most cond_resched will/should be >> removed. Is there any reason why you are not using full preemption when >> requiring low latencies?" >> >> https://lore.kernel.org/r/aoRnUxUlgf_kRlm8@tiehlicka > > That's pretty bad behavior and we might want to fix it in earlier > kernels. Is PREEMPT_NONE effectively dead in 6.18.x and its > existing users? > > If yes, we do want to fix older kernels then we should merge this. > Okay, but that would be stable-only fixes? -- Cheers, David