From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1794236462; b=ykyiKMddrIQC8OcOZLTzzXXaQTeakZy80+0oqioaXKtn+EbgJ/iRHJ4PQs0MavpX0hk4 jQCDJSNM5C32cE8fmDxe6+sgTeFN08qSmy3I6WdjZxVITfBniXeItKB0za0WvGkrWOdph WMqP2l2Y8++eqNJJ932ZS77TrSoFyN9HBLYryx0CkZibBhg47hjyrv/lHld4zX4KXsKqD 9N0KKLSbdu2e6Jx5CsuQUE8NZdgzIWf9RsOThcN+Zwa5KooZkwi7umov8lYfE4FaTNJkd 0LZkT5olVGf7q2Vgk8gpQwzBVXV1zZy7+m0WHKNWDS4uhuwHmk41N2LV5t+a17QScjA== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1794236462; h=Received:DKIM-Signature:Message-ID:Date:MIME-Version:User-Agent: Subject:To:Cc:References:From:Content-Language:Autocrypt:In-Reply-To: Content-Type:Content-Transfer-Encoding; bh=uh98auUTZPUD0WL8RdCfHHJjBpICoJEBbXmih9wp9i0=; b=qBk0mRbkN/uMZxlxIeCsgRk+y0TxKvtjOoUspReVB6gmoGTpXXW4xKtXV8qeCsGlYjZn 9Anb4c2nafJJQJNjz/1heJzRzMHGKblruqzBi/JJa75EQTb/KpdTNDVsz+C7Hdgbdy0Lj CgcAyEnJeZ/yDHLNqfYoiRncmMUholdzWgAxtXPmwgjfi/lEv4IzE2+hH+HMpQwPFJEKz Aq7PWOPE9G9WTY0XAPibS/mk4nzLN+LGb+Z6G/sWgyPNBLEGdDzVkm1MmLh0SlGBoll/v uxvCk1+JTqd1v+BAlwSeZiJ4W8KtlLNt3RAcf6T+D8Aoh2sK/fV8n3+ojN5+hcGWBsA== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b="d/M47nMI"; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id C4A9B606A326 for ; Mon, 9 Nov 2026 15:01:01 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=d/M47nMI Received: by smtp.kernel.org (Postfix) with ESMTPSA id 2E3FA1F008A1; Mon, 9 Nov 2026 15:00:57 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794236461; bh=uh98auUTZPUD0WL8RdCfHHJjBpICoJEBbXmih9wp9i0=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=d/M47nMIJR7aihTAaLZTBAlDzN54La0m6muqe34XXaAoUlvwToPrvviy1SqgkXLMc YPTGC8eDZspkkjxHE4mcuSsymqau53tKpMWsFYubYX1NKW3zQechkdk1CALfIaVZn0 tHQEU3tcG89EfTeTfjio/VUFU3JPBIjNOGvuGJoxMIS6dTAI5c71rtjg3NblXoUdyE mbRVq454n0Tzo/+QEQg174qR/Lu+6SKfv3Fs72d+0wS56avAdt2JobeNrCvKfbXAp9 pL69rUuWvwSjF4IN8k4YE2Apd1o3FAapcf+MukXRCqaKvadvemojGqRmqopClB7Ulh hWrqWiGrkS8dQ== Message-ID: <0fb93821-3449-44f5-936b-986b485b5f9f@kernel.org> Date: Mon, 9 Nov 2026 16:00:55 +0100 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 3/3] mm/khugepaged: release contended mmap read lock after scanning enough VMAs To: "Lorenzo Stoakes (ARM)" , Kiryl Shutsemau Cc: Zi Yan , Baolin Wang , "Liam R. Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20261105-khugepaged-latency-fix-v2-0-4384bbfe4606@kernel.org> <20261105-khugepaged-latency-fix-v2-3-4384bbfe4606@kernel.org> <8ffe5804-a8a1-4486-b5eb-7ce5f451b77c@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzS5EYXZpZCBIaWxk ZW5icmFuZCAoQ3VycmVudCkgPGRhdmlkQGtlcm5lbC5vcmc+wsGQBBMBCAA6AhsDBQkmWAik AgsJBBUKCQgCFgICHgUCF4AWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaYJt/AIZAQAKCRBN 3hD3AP+DWriiD/9BLGEKG+N8L2AXhikJg6YmXom9ytRwPqDgpHpVg2xdhopoWdMRXjzOrIKD g4LSnFaKneQD0hZhoArEeamG5tyo32xoRsPwkbpIzL0OKSZ8G6mVbFGpjmyDLQCAxteXCLXz ZI0VbsuJKelYnKcXWOIndOrNRvE5eoOfTt2XfBnAapxMYY2IsV+qaUXlO63GgfIOg8RBaj7x 3NxkI3rV0SHhI4GU9K6jCvGghxeS1QX6L/XI9mfAYaIwGy5B68kF26piAVYv/QZDEVIpo3t7 /fjSpxKT8plJH6rhhR0epy8dWRHk3qT5tk2P85twasdloWtkMZ7FsCJRKWscm1BLpsDn6EQ4 jeMHECiY9kGKKi8dQpv3FRyo2QApZ49NNDbwcR0ZndK0XFo15iH708H5Qja/8TuXCwnPWAcJ DQoNIDFyaxe26Rx3ZwUkRALa3iPcVjE0//TrQ4KnFf+lMBSrS33xDDBfevW9+Dk6IISmDH1R HFq2jpkN+FX/PE8eVhV68B2DsAPZ5rUwyCKUXPTJ/irrCCmAAb5Jpv11S7hUSpqtM/6oVESC 3z/7CzrVtRODzLtNgV4r5EI+wAv/3PgJLlMwgJM90Fb3CB2IgbxhjvmB1WNdvXACVydx55V7 LPPKodSTF29rlnQAf9HLgCphuuSrrPn5VQDaYZl4N/7zc2wcWM7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 11/9/26 15:12, Lorenzo Stoakes (ARM) wrote: > On Mon, Nov 09, 2026 at 03:01:29PM +0100, David Hildenbrand (Arm) wrote: >> On 11/6/26 13:07, Lorenzo Stoakes (ARM) wrote: >>> On reflection, I think this would be better implemented as something like: >>> >>> static void collapse_scan_mm_slot(unsigned int progress_max, ...) >>> { >>> unsigned int long lock_start = jiffies; >>> ... >>> for_each_vma(vmi, vma) { >>> ... >>> if (time_after(jiffies, lock_start) && mmap_lock_is_contended(mm)) { >>> mmap_read_unlock(mm); >>> cond_resched(); >>> mmap_read_lock(mm); >>> lock_start = jiffies; >>> vma_iter_set(&vmi, mm_slot->address); >>> continue; >>> } >>> } >>> } >>> >>> Which avoids magic numbers, and means if we hold the lock for 1-10ms (depending >>> on HZ), which is a long time in the kernel to hold a lock, AND the lock is >>> contended, we drop/reacquire. >> >> That approach indeed sounds better to me regarding lock contention. > > Unfortunately it makes things measurably worse (it's not fine-grained > enough). Trying to get a fine-grained timer will involve preemption > disable/enable pairs, so a no go. Ah. :( > > This is the alternative I have locally which explains the number, puts VMA in > the name and it applies to _all_ VMAs as requested. > > Let me know what you think! > > The jiffies version would obviously just be this but with the above, but that > one results in max ~2 ms-level stalls rather than ~200us. > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index 7d1414a9b2a5..59bbef61f104 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -66,6 +66,13 @@ static DEFINE_READ_MOSTLY_HASHTABLE(mm_slots_hash, MM_SLOTS_HASH_BITS); > > static struct kmem_cache *mm_slot_cache __ro_after_init; > > +/* > + * The number of VMAs to walk before checking for, and relieving, mmap lock > + * contention. This bounds the maximum mmap read lock to an estimated ~100 - > + * 300us depending on hardware. > + */ > +#define KHUGEPAGED_MAX_VMA_SCAN_BATCH 1024 > + > /** > * struct khugepaged_mm_slot - khugepaged information per mm that is being scanned > * @slot: hash lookup from mm to mm_slot > @@ -2910,6 +2917,7 @@ static void collapse_scan_mm_slot(unsigned int progress_max, > struct mm_struct *mm; > struct vm_area_struct *vma; > unsigned int progress_prev = cc->progress; > + unsigned int nr_vmas_batch = 0; > > lockdep_assert_held(&khugepaged_mm_lock); > *result = SCAN_FAIL; > @@ -2942,6 +2950,18 @@ static void collapse_scan_mm_slot(unsigned int progress_max, > for_each_vma(vmi, vma) { > unsigned long hstart, hend; > > + /* Relieve mmap lock contention after walking a batch of VMAs. */ > + if (nr_vmas_batch++ >= KHUGEPAGED_MAX_VMA_SCAN_BATCH) { > + nr_vmas_batch = 0; > + if (mmap_lock_is_contended(mm)) { > + mmap_read_unlock(mm); > + cond_resched(); > + mmap_read_lock(mm); > + vma_iter_set(&vmi, mm_slot->address); > + continue; > + } > + } > + > cond_resched(); > if (unlikely(collapse_test_exit_or_disable(mm))) { > cc->progress++; Looks better to me than the original. @Kiryl, we talked about dropping the mmap lock recently, would this create a regression for your scanning use-cases? -- Cheers, David