From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-3.8 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 28616C04AB4 for ; Fri, 17 May 2019 11:55:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id E5F2720815 for ; Fri, 17 May 2019 11:55:05 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="OTigpAmA" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728611AbfEQLzE (ORCPT ); Fri, 17 May 2019 07:55:04 -0400 Received: from bombadil.infradead.org ([198.137.202.133]:43846 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727811AbfEQLzE (ORCPT ); Fri, 17 May 2019 07:55:04 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20170209; h=Content-Type:MIME-Version:References: Subject:Cc:To:From:Date:Message-Id:Sender:Reply-To:Content-Transfer-Encoding: Content-ID:Content-Description:Resent-Date:Resent-From:Resent-Sender: Resent-To:Resent-Cc:Resent-Message-ID:In-Reply-To:List-Id:List-Help: List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=BB+a1yAjmGBaCcsmFlZZE8gT8m2E1PyHUTm3q9L2zV8=; b=OTigpAmAdL7mg9vDisVbcjE4G0 pPbA5jd9aZKd4FP0X7whbf/A2shmtw7IA0uCf+L2FWO0/nmbgSs2feGC9D+ipz+w9aO/tW5tpKgHC z6rHkCo6wJd7SlT2W1unKAYDuGH9f9k5Ww6uYCh+18/RBOAvo5yiB6iWsBivOQAbPxPgo40mhTWyd h2iUWYwhHBFnBWcaQCTag4f9nq711zbvzXSxiscmxL2r9XbMwKZRkNvgpvAmp74f4wxWsMmteQPlw 3HAAZ/aMVpi2REoHtNgRgoRRxRzWVM+j+xGYzz9DUFmtbUfHLM4wV171SrcAyOvBAqdYXfmzi6vyo vODmVk1A==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=hirez.programming.kicks-ass.net) by bombadil.infradead.org with esmtpsa (Exim 4.90_1 #2 (Red Hat Linux)) id 1hRbRy-00022T-W1; Fri, 17 May 2019 11:54:59 +0000 Received: by hirez.programming.kicks-ass.net (Postfix, from userid 0) id C1EF62027B748; Fri, 17 May 2019 13:54:56 +0200 (CEST) Message-Id: <20190517115418.481392777@infradead.org> User-Agent: quilt/0.65 Date: Fri, 17 May 2019 13:52:34 +0200 From: Peter Zijlstra To: peterz@infradead.org, mingo@kernel.org Cc: yabinc@google.com, acme@kernel.org, alexander.shishkin@linux.intel.com, jolsa@redhat.com, namhyung@kernel.org, mark.rutland@arm.com, linux-kernel@vger.kernel.org Subject: [PATCH 4/4] perf/ring-buffer: Use regular variables for nesting References: <20190517115230.437269790@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org While the IRQ/NMI will nest, the nest-count will be invariant over the actual exception, since it will decrement equal to increment. This means we can -- carefully -- use a regular variable since the typical LOAD-STORE race doesn't exist (similar to preempt_count). This optimizes the ring-buffer for all LOAD-STORE architectures, since they need to use atomic ops to implement local_t. Suggested-by: Alexander Shishkin Signed-off-by: Peter Zijlstra (Intel) --- kernel/events/internal.h | 4 ++-- kernel/events/ring_buffer.c | 41 ++++++++++++++++++++++++++--------------- 2 files changed, 28 insertions(+), 17 deletions(-) --- a/kernel/events/internal.h +++ b/kernel/events/internal.h @@ -24,7 +24,7 @@ struct ring_buffer { atomic_t poll; /* POLL_ for wakeups */ local_t head; /* write position */ - local_t nest; /* nested writers */ + unsigned int nest; /* nested writers */ local_t events; /* event limit */ local_t wakeup; /* wakeup stamp */ local_t lost; /* nr records lost */ @@ -41,7 +41,7 @@ struct ring_buffer { /* AUX area */ long aux_head; - local_t aux_nest; + unsigned int aux_nest; long aux_wakeup; /* last aux_watermark boundary crossed by aux_head */ unsigned long aux_pgoff; int aux_nr_pages; --- a/kernel/events/ring_buffer.c +++ b/kernel/events/ring_buffer.c @@ -38,7 +38,12 @@ static void perf_output_get_handle(struc struct ring_buffer *rb = handle->rb; preempt_disable(); - local_inc(&rb->nest); + + /* + * Avoid an explicit LOAD/STORE such that architectures with memops + * can use them. + */ + (*(volatile unsigned int *)&rb->nest)++; handle->wakeup = local_read(&rb->wakeup); } @@ -46,6 +51,17 @@ static void perf_output_put_handle(struc { struct ring_buffer *rb = handle->rb; unsigned long head; + unsigned int nest; + + /* + * If this isn't the outermost nesting, we don't have to update + * @rb->user_page->data_head. + */ + nest = READ_ONCE(rb->nest); + if (nest > 1) { + WRITE_ONCE(rb->nest, nest - 1); + goto out; + } again: /* @@ -65,15 +81,6 @@ static void perf_output_put_handle(struc */ /* - * If this isn't the outermost nesting, we don't have to update - * @rb->user_page->data_head. - */ - if (local_read(&rb->nest) > 1) { - local_dec(&rb->nest); - goto out; - } - - /* * Since the mmap() consumer (userspace) can run on a different CPU: * * kernel user @@ -108,7 +115,7 @@ static void perf_output_put_handle(struc * write will (temporarily) publish a stale value. */ barrier(); - local_set(&rb->nest, 0); + WRITE_ONCE(rb->nest, 0); /* * Ensure we decrement @rb->nest before we validate the @rb->head. @@ -116,7 +123,7 @@ static void perf_output_put_handle(struc */ barrier(); if (unlikely(head != local_read(&rb->head))) { - local_inc(&rb->nest); + WRITE_ONCE(rb->nest, 1); goto again; } @@ -355,6 +362,7 @@ void *perf_aux_output_begin(struct perf_ struct perf_event *output_event = event; unsigned long aux_head, aux_tail; struct ring_buffer *rb; + unsigned int nest; if (output_event->parent) output_event = output_event->parent; @@ -385,13 +393,16 @@ void *perf_aux_output_begin(struct perf_ if (!refcount_inc_not_zero(&rb->aux_refcount)) goto err; + nest = READ_ONCE(rb->aux_nest); /* * Nesting is not supported for AUX area, make sure nested * writers are caught early */ - if (WARN_ON_ONCE(local_xchg(&rb->aux_nest, 1))) + if (WARN_ON_ONCE(nest)) goto err_put; + WRITE_ONCE(rb->aux_nest, nest + 1); + aux_head = rb->aux_head; handle->rb = rb; @@ -419,7 +430,7 @@ void *perf_aux_output_begin(struct perf_ if (!handle->size) { /* A, matches D */ event->pending_disable = smp_processor_id(); perf_output_wakeup(handle); - local_set(&rb->aux_nest, 0); + WRITE_ONCE(rb->aux_nest, 0); goto err_put; } } @@ -508,7 +519,7 @@ void perf_aux_output_end(struct perf_out handle->event = NULL; - local_set(&rb->aux_nest, 0); + WRITE_ONCE(rb->aux_nest, 0); /* can't be last */ rb_free_aux(rb); ring_buffer_put(rb);