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Tue, 4 Aug 2026 20:11:49 +0000 (GMT) Message-ID: <2a9aec78-fe1b-49c9-884f-a59eb36c8905@linux.ibm.com> Date: Wed, 5 Aug 2026 01:41:48 +0530 Precedence: bulk X-Mailing-List: rust-for-linux@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 10/17] preempt: Introduce HAS_SEPARATE_PREEMPT_RESCHED_BITS To: Boqun Feng , Peter Zijlstra Cc: Ingo Molnar , Will Deacon , Waiman Long , Gary Guo , Alice Ryhl , Lyude Paul , Daniel Almeida , =?UTF-8?Q?Onur_=C3=96zkan?= , Miguel Ojeda , Danilo Krummrich , linux-kernel@vger.kernel.org, rust-for-linux@vger.kernel.org References: <20260804161447.84806-1-boqun@kernel.org> <20260804161447.84806-11-boqun@kernel.org> Content-Language: en-US From: Shrikanth Hegde In-Reply-To: <20260804161447.84806-11-boqun@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-Reinject: loops=2 maxloops=12 X-Proofpoint-ORIG-GUID: Ens6O40a7TE4yzmYogM-L86PRSJ_GViP X-Proofpoint-Spam-Info: AW1haW4tMjYwODA0MDE2MCBTYWx0ZWRfX+QDHBLCczLTB 2N6J5++bGOByXufmDmaJ8UAwAkMobeNmnZLmjwPGuZHFE1wxN0K2ZmHlPt+CRbm6pzuyr3tFAFL hH5cTPlOACqgeNyaFB1WWBaEl32GsnM= X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwODA0MDE2MCBTYWx0ZWRfX4227pYUAY40c Tv+p+TGPxGppqWDyOQUr6Rp5ttK8D2l5WjbGuvMILcbVLqZbXnOadgjZJclwyOafOByv+5vXtBB F/Sl4gJy8hpNws2vkiugXW/jfScbJQCvTaVuuNDFuLKMMXyyyOU8zzBKzvKpmnzo800ivjPEZ7F 3ZmDKJUDBnoiu/O+5arVVb4avLZqHU2wpN2PoyGB3XkHGoNt6LKJILjT5k8vBy2Gu4r8HzG6l2G HJJYbyNRJD6Ta3VwZh1rwmKAY9PjUqavpDwpe/0koG9fcXFxdcViKRpqTI6k2DLkuVNBRH3jedo 3HpxK0i2Czd8juN6rLnJ8w5Pg7JwIJNG2Jm/hC7cXCF1eiSg6olVOJLY22s2tC6pwUzwZfQJs8b wXnMKXRWEXmz+41vzk/ZzNwvwJrtupulfTpDDdpZVdCzFq8tzq2wrFAc5kmIllY+JPfNb0SFgak VioM+OvfAwnEZunvKZw== X-Proofpoint-GUID: 7PjqqsxZLKef0b0JjLCFoG1bTFFDuvM8 X-Authority-Analysis: v=2.4 cv=KfzidwYD c=1 sm=1 tr=0 ts=6a72478c cx=c_pps a=3Bg1Hr4SwmMryq2xdFQyZA==:117 a=3Bg1Hr4SwmMryq2xdFQyZA==:17 a=IkcTkHD0fZMA:10 a=Sv0fKeRqtYgA:10 a=VkNPw1HP01LnGYTKEx00:22 a=RnoormkPH1_aCDwRdu11:22 a=RzCfie-kr_QcCd8fBx8p:22 a=8qcXNv606LjFGKxQDKUA:9 a=QEXdDO2ut3YA:10 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1176,Hydra:6.1.134,FMLib:17.12.100.49 definitions=2026-08-04_05,2026-08-04_02,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 lowpriorityscore=0 bulkscore=0 impostorscore=0 suspectscore=0 malwarescore=0 adultscore=0 clxscore=1011 priorityscore=1501 phishscore=0 spamscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2606150000 definitions=main-2608040160 Hi Boqun, On 8/4/26 9:44 PM, Boqun Feng wrote: > With the changes that enable preempt count to track IRQ disabling > nesting, we don't have enough bits in 32-bit preempt count > implementation, as a result we move NMI nesting bits out of the 32-bit > preempt count. However on the architectures that can support 64-bit > preempt count implementation, we can keep the NMI nesting bits in the > 32-bit preempt count and avoid maintaining NMI nesting bits outside of > the same cache line. > [...] > --- a/include/linux/hardirq.h > +++ b/include/linux/hardirq.h > @@ -10,8 +10,6 @@ > #include > #include > > -DECLARE_PER_CPU(unsigned int, nmi_nesting); > - > extern void synchronize_irq(unsigned int irq); > extern bool synchronize_hardirq(unsigned int irq); > > @@ -94,6 +92,37 @@ void irq_exit_rcu(void); > #define arch_nmi_exit() do { } while (0) > #endif > > +#ifdef CONFIG_HAS_SEPARATE_PREEMPT_RESCHED_BITS > +static __always_inline void __preempt_count_nmi_enter(void) > +{ > + __preempt_count_add(NMI_OFFSET + HARDIRQ_OFFSET); > +} > + > +static __always_inline void __preempt_count_nmi_exit(void) > +{ > + __preempt_count_sub(NMI_OFFSET + HARDIRQ_OFFSET); > +} > +#else > +DECLARE_PER_CPU(unsigned int, nmi_nesting); > + > +#define __preempt_count_nmi_enter() \ > + do { \ > + __preempt_count_add(HARDIRQ_OFFSET); \ nit: This limit is because to have the same behavior as other case when NMI_BITS=4 right? It is not easy to infer that from comment. > + /* Maximum NMI nesting is 15. */ \ > + BUG_ON(__this_cpu_read(nmi_nesting) >= 15); \ > + __this_cpu_inc(nmi_nesting); \ > + preempt_count_set(preempt_count() | NMI_MASK); \ Is there a reason preempt count updates are split rather than folded into a single preempt_count update? > + } while (0) > + > +#define __preempt_count_nmi_exit() \ > + do { \ > + __preempt_count_sub(HARDIRQ_OFFSET); \ > + if (!__this_cpu_dec_return(nmi_nesting)) \ > + preempt_count_set(preempt_count() & ~NMI_MASK); \ > + } while (0) > + > +#endif > + > /* > * NMI vs Tracing > * -------------- > @@ -110,18 +139,14 @@ void irq_exit_rcu(void); > do { \ > lockdep_off(); \ > arch_nmi_enter(); \ > - /* Maximum NMI nesting is 15. */ \ > - BUG_ON(__this_cpu_read(nmi_nesting) >= 15); \ > - __this_cpu_inc(nmi_nesting); \ > - __preempt_count_add(HARDIRQ_OFFSET); \ > - preempt_count_set(preempt_count() | NMI_MASK); \ > + __preempt_count_nmi_enter(); \ > } while (0) > > #define nmi_enter() \ > do { \ > __nmi_enter(); \ > lockdep_hardirq_enter(); \ > - ct_nmi_enter(); \ > + ct_nmi_enter(); \ > instrumentation_begin(); \ > ftrace_nmi_enter(); \ > instrumentation_end(); \ > @@ -129,12 +154,8 @@ void irq_exit_rcu(void); > > #define __nmi_exit() \ > do { \ > - unsigned int nesting; \ > BUG_ON(!in_nmi()); \ > - __preempt_count_sub(HARDIRQ_OFFSET); \ > - nesting = __this_cpu_dec_return(nmi_nesting); \ > - if (!nesting) \ > - preempt_count_set(preempt_count() & ~NMI_MASK); \ > + __preempt_count_nmi_exit(); \ > arch_nmi_exit(); \ > lockdep_on(); \ > } while (0)