From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-wm1-f74.google.com (mail-wm1-f74.google.com [209.85.128.74]) (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 3BB3E342C8F for ; Thu, 19 Feb 2026 15:03:24 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.128.74 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1771513406; cv=none; b=dg107dOiqwwrC6ekRbfYC/B8drFDW7QwKrAiNRq0aWSj16xBkMd8LPPt2u7stYaSc6o/ZfKKIJ3RJ//RA7pZeZ7hWlkSJr7AsauVysTKIe+CgKhFEOVMvuPD5tvfaopIgcmZqgMVob8Owk1JUX64PPF7rwv9feoTmAHbqX0pkn8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1771513406; c=relaxed/simple; bh=dpvGTQyDqc4E13kT5zotIDIaJ5WjPgu62HiL2EFpWdo=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=iGry6IVzj6Ctjq1YN3pTiB5LuB4lVqsJmzblOWOv0nu1FhTGaHhlhJCVxjGeivwyn9wbYn16iTMSQB4aebHaSlbQ00rg0d5EX/HriPa5GidcxWinlM6hhbkovZoIMX9j51Iwd8yZ5f8gMB1j7TLDoBu0PVwEojHy3dV79Qec4KE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com; spf=pass smtp.mailfrom=flex--vdonnefort.bounces.google.com; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b=1ktmX1a7; arc=none smtp.client-ip=209.85.128.74 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=flex--vdonnefort.bounces.google.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="1ktmX1a7" Received: by mail-wm1-f74.google.com with SMTP id 5b1f17b1804b1-482d8e6e13aso7001245e9.3 for ; Thu, 19 Feb 2026 07:03:24 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1771513403; x=1772118203; darn=vger.kernel.org; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:from:to:cc:subject:date:message-id:reply-to; bh=CJWFRtBApZCpkTiQfcHr+n+DXKxoodOPjTOPKitJmqA=; b=1ktmX1a7Tb3JShDzRDVDdWEbfhr7WZTvx1cUGnl5klNx1vL8OoQ5v6bEuaIqOrCJal DgTLNpdeNgOPUvx3WRnpDiFcYimCaNqQfgvNifIP9R4zbURYhSlFfxPZsx2WtcxKBUYs Hsfg75CiHVhX/9g56aqHzHd6WEyz8CIG8I7Ol5L1/3E2TmqC6mLJWOL+aq/1G33GbzuS Lg8uFmhNKdnK2xkAWqSiHBrF2PR9BkAFdZKu33y4ZrjB2iZmeuOYCfnOxyw9JLYuZ3OE u97xdqr/W0kqnszy4xXcKp2qK5ppjEbCRSgqm5xKM905kohZNwtB2Kko1POlwgRdfenf /07g== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1771513403; x=1772118203; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=CJWFRtBApZCpkTiQfcHr+n+DXKxoodOPjTOPKitJmqA=; b=r2ZH0nGXpZIVRBinW6oN/vv6G3G07y8zxhJ3or3XPFf5kyOrKEMWFPcznKZO5ie4ou ROHUd4g+NXiaZ57aFgxnc7f5ISQVRa55ap+nKIMhzwdBDdzIgTgWQ8cTYymNokGmTYbJ 9Te33ObtrW062vwKYXTDmEovaVbElsMFIbvtRRG99o8bSfX2W0AMkox/662TFlPFS1WG /Lv+/IR5mAyZ1s3WSplGPQoYV6OE8rh2YC3gXoS2vKQCln+Isknj/vhW+MEzQuXE5hjc z7qsoAlAHnzKz7CsPGEUtPxXX4gmlBtlHM8Ku2QdZ3CeTTBfW2uSaTwA+8ePxHO/9WFB UAYw== X-Forwarded-Encrypted: i=1; AJvYcCWjIolasg3aD0gDAbnzqMNBwoFddsqfhYlSfQtXDDifowLBYhR7Vvb/1OR9aXqx4+kJfMd5LIisTnETqbpwYcCUEcI=@vger.kernel.org X-Gm-Message-State: AOJu0YwpVWfXLscBGXVHOK42G98mYHy9il1iIyUT55oryX2e0QF8gpsS 4UIZREYy0rD/cszvebZTlBqWuQsWE0N6M6xgvjpUUcIYHDuHed8XM5wguZbRCi7QOdZG1yg7caY HVnJtxJSwF5FzDF9GP6sz/Q== X-Received: from wmqf8.prod.google.com ([2002:a05:600c:4e88:b0:483:509d:729b]) (user=vdonnefort job=prod-delivery.src-stubby-dispatcher) by 2002:a05:600c:a51:b0:480:1e40:3d2 with SMTP id 5b1f17b1804b1-4839e6682a2mr43557455e9.29.1771513402562; Thu, 19 Feb 2026 07:03:22 -0800 (PST) Date: Thu, 19 Feb 2026 15:02:41 +0000 In-Reply-To: <20260219150307.14538-1-vdonnefort@google.com> Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20260219150307.14538-1-vdonnefort@google.com> X-Mailer: git-send-email 2.53.0.335.g19a08e0c02-goog Message-ID: <20260219150307.14538-5-vdonnefort@google.com> Subject: [PATCH v12 04/30] ring-buffer: Add non-consuming read for ring-buffer remotes From: Vincent Donnefort To: rostedt@goodmis.org, mhiramat@kernel.org, mathieu.desnoyers@efficios.com, linux-trace-kernel@vger.kernel.org, maz@kernel.org, oliver.upton@linux.dev, joey.gouly@arm.com, suzuki.poulose@arm.com, yuzenghui@huawei.com Cc: kvmarm@lists.linux.dev, linux-arm-kernel@lists.infradead.org, jstultz@google.com, qperret@google.com, will@kernel.org, aneesh.kumar@kernel.org, kernel-team@android.com, linux-kernel@vger.kernel.org, Vincent Donnefort Content-Type: text/plain; charset="UTF-8" Hopefully, the remote will only swap pages on the kernel instruction (via the swap_reader_page() callback). This means we know at what point the ring-buffer geometry has changed. It is therefore possible to rearrange the kernel view of that ring-buffer to allow non-consuming read. Reviewed-by: Steven Rostedt (Google) Signed-off-by: Vincent Donnefort diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c index cd9e686b212f..e36e70e2c998 100644 --- a/kernel/trace/ring_buffer.c +++ b/kernel/trace/ring_buffer.c @@ -5383,10 +5383,51 @@ static bool rb_read_remote_meta_page(struct ring_buffer_per_cpu *cpu_buffer) return rb_num_of_entries(cpu_buffer); } +static void rb_update_remote_head(struct ring_buffer_per_cpu *cpu_buffer) +{ + struct buffer_page *next, *orig; + int retry = 3; + + orig = next = cpu_buffer->head_page; + rb_inc_page(&next); + + /* Run after the writer */ + while (cpu_buffer->head_page->page->time_stamp > next->page->time_stamp) { + rb_inc_page(&next); + + rb_list_head_clear(cpu_buffer->head_page->list.prev); + rb_inc_page(&cpu_buffer->head_page); + rb_set_list_to_head(cpu_buffer->head_page->list.prev); + + if (cpu_buffer->head_page == orig) { + if (WARN_ON_ONCE(!(--retry))) + return; + } + } + + orig = cpu_buffer->commit_page = cpu_buffer->head_page; + retry = 3; + + while (cpu_buffer->commit_page->page->time_stamp < next->page->time_stamp) { + rb_inc_page(&next); + rb_inc_page(&cpu_buffer->commit_page); + + if (cpu_buffer->commit_page == orig) { + if (WARN_ON_ONCE(!(--retry))) + return; + } + } +} + static void rb_iter_reset(struct ring_buffer_iter *iter) { struct ring_buffer_per_cpu *cpu_buffer = iter->cpu_buffer; + if (cpu_buffer->remote) { + rb_read_remote_meta_page(cpu_buffer); + rb_update_remote_head(cpu_buffer); + } + /* Iterator usage is expected to have record disabled */ iter->head_page = cpu_buffer->reader_page; iter->head = cpu_buffer->reader_page->read; @@ -5539,7 +5580,7 @@ rb_update_iter_read_stamp(struct ring_buffer_iter *iter, static struct buffer_page * __rb_get_reader_page_from_remote(struct ring_buffer_per_cpu *cpu_buffer) { - struct buffer_page *new_reader, *prev_reader; + struct buffer_page *new_reader, *prev_reader, *prev_head, *new_head, *last; if (!rb_read_remote_meta_page(cpu_buffer)) return NULL; @@ -5563,10 +5604,32 @@ __rb_get_reader_page_from_remote(struct ring_buffer_per_cpu *cpu_buffer) WARN_ON_ONCE(prev_reader == new_reader); - cpu_buffer->reader_page->page = new_reader->page; - cpu_buffer->reader_page->id = new_reader->id; - cpu_buffer->reader_page->read = 0; - cpu_buffer->read_stamp = cpu_buffer->reader_page->page->time_stamp; + prev_head = new_reader; /* New reader was also the previous head */ + new_head = prev_head; + rb_inc_page(&new_head); + last = prev_head; + rb_dec_page(&last); + + /* Clear the old HEAD flag */ + rb_list_head_clear(cpu_buffer->head_page->list.prev); + + prev_reader->list.next = prev_head->list.next; + prev_reader->list.prev = prev_head->list.prev; + + /* Swap prev_reader with new_reader */ + last->list.next = &prev_reader->list; + new_head->list.prev = &prev_reader->list; + + new_reader->list.prev = &new_reader->list; + new_reader->list.next = &new_head->list; + + /* Reactivate the HEAD flag */ + rb_set_list_to_head(&last->list); + + cpu_buffer->head_page = new_head; + cpu_buffer->reader_page = new_reader; + cpu_buffer->pages = &new_head->list; + cpu_buffer->read_stamp = new_reader->page->time_stamp; cpu_buffer->lost_events = cpu_buffer->meta_page->reader.lost_events; return rb_page_size(cpu_buffer->reader_page) ? cpu_buffer->reader_page : NULL; @@ -6150,7 +6213,7 @@ ring_buffer_read_start(struct trace_buffer *buffer, int cpu, gfp_t flags) struct ring_buffer_per_cpu *cpu_buffer; struct ring_buffer_iter *iter; - if (!cpumask_test_cpu(cpu, buffer->cpumask) || buffer->remote) + if (!cpumask_test_cpu(cpu, buffer->cpumask)) return NULL; iter = kzalloc(sizeof(*iter), flags); -- 2.53.0.335.g19a08e0c02-goog