[{"id":3675884,"web_url":"http://patchwork.ozlabs.org/comment/3675884/","msgid":"","date":"2026-04-10T13:39:30","subject":"Re: [patch 19/38] kcsan: Replace get_cycles() usage","submitter":{"id":77293,"url":"http://patchwork.ozlabs.org/api/people/77293/","name":"Marco Elver","email":"elver@google.com"},"content":"On Fri, 10 Apr 2026 at 14:20, Thomas Gleixner wrote:\n>\n> KCSAN uses get_cycles() for two purposes:\n>\n> 1) Seeding the random state with get_cycles() is a historical leftover.\n>\n> 2) The microbenchmark uses get_cycles(), which provides an unit less\n> counter value and is not guaranteed to be functional on all\n> systems/platforms.\n>\n> Use random_get_entropy() for seeding the random state and ktime_get() which\n> is universaly functional and provides at least a comprehensible unit.\n>\n> This is part of a larger effort to remove get_cycles() usage from\n> non-architecture code.\n>\n> Signed-off-by: Thomas Gleixner \n> Cc: Marco Elver \n> Cc: Dmitry Vyukov \n> Cc: kasan-dev@googlegroups.com\n\nReviewed-by: Marco Elver \n\n> ---\n> kernel/kcsan/core.c | 2 +-\n> kernel/kcsan/debugfs.c | 8 ++++----\n> 2 files changed, 5 insertions(+), 5 deletions(-)\n>\n> --- a/kernel/kcsan/core.c\n> +++ b/kernel/kcsan/core.c\n> @@ -798,7 +798,7 @@ void __init kcsan_init(void)\n> BUG_ON(!in_task());\n>\n> for_each_possible_cpu(cpu)\n> - per_cpu(kcsan_rand_state, cpu) = (u32)get_cycles();\n> + per_cpu(kcsan_rand_state, cpu) = (u32)random_get_entropy();\n>\n> /*\n> * We are in the init task, and no other tasks should be running;\n> --- a/kernel/kcsan/debugfs.c\n> +++ b/kernel/kcsan/debugfs.c\n> @@ -58,7 +58,7 @@ static noinline void microbenchmark(unsi\n> {\n> const struct kcsan_ctx ctx_save = current->kcsan_ctx;\n> const bool was_enabled = READ_ONCE(kcsan_enabled);\n> - u64 cycles;\n> + ktime_t nsecs;\n>\n> /* We may have been called from an atomic region; reset context. */\n> memset(¤t->kcsan_ctx, 0, sizeof(current->kcsan_ctx));\n> @@ -70,16 +70,16 @@ static noinline void microbenchmark(unsi\n>\n> pr_info(\"%s begin | iters: %lu\\n\", __func__, iters);\n>\n> - cycles = get_cycles();\n> + nsecs = ktime_get();\n> while (iters--) {\n> unsigned long addr = iters & ((PAGE_SIZE << 8) - 1);\n> int type = !(iters & 0x7f) ? KCSAN_ACCESS_ATOMIC :\n> (!(iters & 0xf) ? KCSAN_ACCESS_WRITE : 0);\n> __kcsan_check_access((void *)addr, sizeof(long), type);\n> }\n> - cycles = get_cycles() - cycles;\n> + nsecs = ktime_get() - nsecs;\n>\n> - pr_info(\"%s end | cycles: %llu\\n\", __func__, cycles);\n> + pr_info(\"%s end | nsecs: %llu\\n\", __func__, nsecs);\n>\n> WRITE_ONCE(kcsan_enabled, was_enabled);\n> /* restore context */\n>","headers":{"Return-Path":"\n ","X-Original-To":["incoming@patchwork.ozlabs.org","linuxppc-dev@lists.ozlabs.org"],"Delivered-To":"patchwork-incoming@legolas.ozlabs.org","Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=google.com header.i=@google.com header.a=rsa-sha256\n header.s=20251104 header.b=nVxA9xkL;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=lists.ozlabs.org\n (client-ip=2404:9400:21b9:f100::1; helo=lists.ozlabs.org;\n envelope-from=linuxppc-dev+bounces-19610-incoming=patchwork.ozlabs.org@lists.ozlabs.org;\n receiver=patchwork.ozlabs.org)","lists.ozlabs.org;\n arc=pass smtp.remote-ip=\"2607:f8b0:4864:20::1235\" arc.chain=google.com","lists.ozlabs.org;\n dmarc=pass (p=reject dis=none) header.from=google.com","lists.ozlabs.org;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=google.com header.i=@google.com header.a=rsa-sha256\n header.s=20251104 header.b=nVxA9xkL;\n\tdkim-atps=neutral","lists.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=google.com\n (client-ip=2607:f8b0:4864:20::1235; 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Donenfeld\" ,\n\tRichard Henderson ,\n linux-alpha@vger.kernel.org,\n\tRussell King , linux-arm-kernel@lists.infradead.org,\n\tCatalin Marinas ,\n Huacai Chen ,\n\tloongarch@lists.linux.dev, Geert Uytterhoeven ,\n\tlinux-m68k@lists.linux-m68k.org, Dinh Nguyen ,\n\tJonas Bonn , linux-openrisc@vger.kernel.org,\n\tHelge Deller , linux-parisc@vger.kernel.org,\n\tMichael Ellerman , linuxppc-dev@lists.ozlabs.org,\n\tPaul Walmsley , linux-riscv@lists.infradead.org,\n\tHeiko Carstens , linux-s390@vger.kernel.org,\n\t\"David S. Miller\" , sparclinux@vger.kernel.org","Content-Type":"text/plain; charset=\"UTF-8\"","X-Spam-Status":"No, score=-15.7 required=3.0 tests=ARC_SIGNED,ARC_VALID,\n\tDKIMWL_WL_MED,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,\n\tENV_AND_HDR_SPF_MATCH,RCVD_IN_DNSWL_NONE,SPF_HELO_NONE,SPF_PASS,\n\tUSER_IN_DEF_DKIM_WL,USER_IN_DEF_SPF_WL autolearn=disabled\n\tversion=4.0.1 OzLabs 8","X-Spam-Checker-Version":"SpamAssassin 4.0.1 (2024-03-25) on lists.ozlabs.org"}}]