From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from va-1-115.ptr.blmpb.com (va-1-115.ptr.blmpb.com [209.127.230.115]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 6D8D23F824B for ; Fri, 26 Jun 2026 16:08:04 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.127.230.115 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782490091; cv=none; b=HazmEisUEvEN/WuikGPq/uiZR6yRJ8g2fwrTGK4w2zxPUo7LEHVmlsbCX1SntDDoBvVr+wfxGwO4LOVcra+5gc9jV+dK0+gFtFpLsesmnkMPkhnofkWHRjni9wbuiZ0kA+3CMvcNf7rCE/wzwXcSWJNf9An6HApLFvyL+5QpwX0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782490091; c=relaxed/simple; bh=8NKRGAFGQ+bcV6w6apMk4HHKcRhUlFpWvEX0OlShHnk=; h=To:Subject:References:Content-Type:Cc:Message-Id:Mime-Version: From:Date:In-Reply-To; b=pze97YiO2lYaOtxczxdRW/ftH6sTFZsGzJx18ml2B48IdkEcAuMAX+9ji0+BiqVls7xhebkxAMxksxGYh09t+Ra7wDfrHcuiY00HxmMGI710n07uxEAn6JD8MDoPm02FZ2cyBNRvl8xehQz7uhEVaVL9v9L3ZvrJyQvn5XmhqAw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=bytedance.com; spf=pass smtp.mailfrom=bytedance.com; dkim=pass (2048-bit key) header.d=bytedance.com header.i=@bytedance.com header.b=ELylE378; arc=none smtp.client-ip=209.127.230.115 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=bytedance.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=bytedance.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=bytedance.com header.i=@bytedance.com header.b="ELylE378" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; s=2212171451; d=bytedance.com; t=1782490078; h=from:subject: mime-version:from:date:message-id:subject:to:cc:reply-to:content-type: mime-version:in-reply-to:message-id; bh=zDhBZCBj3yMFOdjYUKyVpvqIhJoCuB5eA4Pwp8bR1FQ=; b=ELylE378Ke92e7r1gPBKQfUqeMHwwGFHrZmsLM7P+MyIbuZUC68zo6A7up2mqPhIossOVr ZJjDt/9Hi0nCy4BMYzqIc6e9R4HOH5SWNQJYzn/Rdzsa1pEPwnj95RxUdVabiAuHkVKShO CGsKeltdS+9snKFIxnxMbRYQRW6GjmyLh7CxXG8VRoGAzmOkLeCVk7mRVAa1IIMxQfc3Sb n81KT81HSHJLeRyXh9nlc2LYcUNMDx8Oa6EJ+BwBpiZp2LN1vbAS0FaxUc9aW2n59QU0Ml TZLpCMHBgs8HCsY0+yocTa4kgnTb8sQ88kVk7wh1pc96PJbDvRv+IhJzfxVwWg== Content-Transfer-Encoding: 7bit X-Lms-Return-Path: To: "Thomas Gleixner" , , , , , , , , , , , , , Subject: Re: [PATCH v8 04/14] smp: Use task-local IPI cpumask in smp_call_function_many_cond() X-Original-From: Chuyi Zhou References: <20260616111127.966468-1-zhouchuyi@bytedance.com> <20260616111127.966468-5-zhouchuyi@bytedance.com> <871pdtjryo.ffs@fw13> <8d3587e6-e3a1-40f5-ba0d-65583a2f1ecb@bytedance.com> Content-Type: text/plain; charset=UTF-8 Cc: Message-Id: <52fc3658-01ef-4138-ba06-65a5be991372@bytedance.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 User-Agent: Mozilla Thunderbird From: "Chuyi Zhou" Date: Sat, 27 Jun 2026 00:07:36 +0800 In-Reply-To: <8d3587e6-e3a1-40f5-ba0d-65583a2f1ecb@bytedance.com> On 2026-06-26 11:47 p.m., Chuyi Zhou wrote: > On 2026-06-26 10:29 p.m., Thomas Gleixner wrote: >> On Tue, Jun 16 2026 at 19:11, Chuyi Zhou wrote: >>> This patch prepares the task-local IPI cpumask during thread creation, and >>> uses the local cpumask to replace the percpu cfd cpumask in >>> smp_call_function_many_cond(). We will enable preemption during >>> csd_lock_wait() later, and this can prevent concurrent access to the >>> cfd->cpumask from other tasks on the current CPU. For cases where >>> cpumask_size() is smaller than or equal to the pointer size, it tries to >>> stash the cpumask in the pointer itself to avoid extra memory allocations. >> >> This one fails the comprehensible test and also does not match the rules of >> how change logs should be written. >> >>> +#if defined(CONFIG_SMP) && defined(CONFIG_PREEMPTION) >>> + union { >>> + cpumask_t *ipi_mask_ptr; >>> + unsigned long ipi_mask_val; >> >> Indentation of the variable name wants TABs not spaces >> >>> @@ -933,10 +934,14 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node) >>> #endif >>> account_kernel_stack(tsk, 1); >>> >>> - err = scs_prepare(tsk, node); >>> + err = smp_task_ipi_mask_alloc(tsk); >> >> Hrm. So we unconditionally allocate another per task CPU mask. How many >> task actually utilize it? >> >> We keep making task_struct and the related things larger every other >> release without actually looking at the resulting overall memory >> consumption. >> > > Thanks, this is a fair concern. > > The task-local cpumask approach came from the earlier discussion with > Sebastian and Nadav. The problem we tried to solve there was the > lifetime of the wait mask once the later patch re-enables preemption > before csd_lock_wait(). At that point the wait mask can no longer be the > per-CPU cfd->cpumask: the task may be preempted or migrate while it is > still iterating the mask, and another task running on the original CPU > could enter smp_call_function_many_cond() and reuse that per-CPU mask. > > I agree that the memory cost needs to be called out explicitly. The > current implementation trades one task-local cpumask for a stable mask > lifetime and avoids adding allocation/failure handling to the generic > IPI path. > > I considered avoiding the fork-time allocation, but the alternatives do > not look straightforward: > > - stack storage is not suitable for large NR_CPUS/CPUMASK_OFFSTACK > configurations; > > - per-CPU storage is exactly what becomes unsafe once the wait is made > preemptible; > > - allocating the mask in smp_call_function_many_cond() would put an > allocation in the generic IPI path. It also cannot rely on a sleeping > allocation because this function is entered from contexts which have > historically only required preemption to be disabled. Using GFP_ATOMIC > would need a failure/fallback path, in which case the latency > improvement becomes opportunistic rather than guaranteed. > > For the motivating x86 TLB flush paths, the users are also not a small > static set of tasks. Ordinary tasks can hit this through exit, unmap, > reclaim, etc., so I do not see a clean way to allocate this only for a > pre-identifiable subset of tasks. To put some numbers around the memory side: On my current x86-64 build, task_struct is 3264 bytes. The patch adds one word to task_struct. With NR_CPUS <= 64 on 64-bit, cpumask_size() fits in that word, so there is no separate allocation. The worst case is the large CPU configuration. With NR_CPUS=8192, cpumask_size() is 1024 bytes, so this becomes one extra 1KiB allocation per task, plus the word in task_struct. That is a real cost, especially on systems with many tasks, and I should document it explicitly. This was also part of the earlier discussion with Sebastian and Nadav. Embedding a plain cpumask_t would have made task_struct grow by 1KiB in that configuration, so the approach here was to keep only one word in task_struct, store the mask inline when it fits, and otherwise allocate only cpumask_size() while creating the task. For comparison, x86 already has several KiB of per-task arch/FPU state. On my current build, struct fpu is 4224 bytes. That does not make the extra cpumask free, but it puts the 8192-CPU worst case in context.